In view of the limited installation space of the dual cross-shaft universal joint steering drive shaft for automobiles, it is difficult to meet the conditions of constant speed transmission, and the phase angle needs to be adjusted to reduce the speed fluctuations. The method of spatial geometric projection is adopted to establish a double cross-shaft universal The speed ratio and rotation angle equations of the steering shaft of the joint bearing model are verified by the constant speed condition. Quantitative analysis of the influence of the phase angle on the speed ratio shows that when the two transmission angles are equal and the phase angle is equal to the included angle of the transmission surface, a constant speed transmission can be achieved between the output shaft and the input shaft; the two transmission angles are not equal When the double cross-shaft universal joint drive shaft cannot achieve constant speed transmission, when the phase angle is equal to the included angle of the transmission surface, the speed difference between the output shaft and the input shaft is the smallest. Therefore, the best phase angle is when the phase angle of the transmission shaft and the transmission surface are equal and opposite in direction. Then, the formula for solving the optimal phase angle based on the hard points of the steering system is deduced, and the calculation program is compiled, and the accuracy of the calculation result of the program is verified through an example. The analysis conclusion and the compiled program have theoretical and practical significance for the design of automobile steering system and the transmission research of other types of universal joints. Keywords phase angle constant velocity transmission cross shaft universal joint steering transmission shaft hard point. Influence Analysis of Phase Angle of Double Cross Universal Joint Steering Transmission Shaft Li Ning1，2 Li Yourong1 Zhou Sizhu2 Zeng Yunyun2( 1 The Key Laboratory of Metallurgical Equipment and Control of Education Ministry, Wuhan University of Science and Technology, Wuhan 430081, China)( 2 Institute of Strength and Vibration of Mechanical Structures, Yangtze University, Jing zhou 434023, China) Abstract Aiming at the problem of the installation space of double cross universal joint steering transmissionshaft is limited, it is difficult to meet the condition of constant speed drive, and need to adjust the phase angle to reduce the rotating speed fluctuation, the rotating speed ratio and rotating angle equations of steering transmission shaft are established by means of spatial geometric projection method, and the equation is verified by using the constant velocity condition. The influence of phase angle on rotating speed ratio are quantitatively analyzed, the results show that the input shaft and output shaft can realize constant speed drive while two transmissionangle are equal and the phase angle is equal to the transmission plane angle, and the double cross universal joint transmission shaft can't realize the constant speed drive while two transmission angle are not equal. However, the speed difference of input shaft and output shaft is the smallest while the phase angle is equal to the transmission plane angle. Therefore, this is the best phase angle when phase angle is equal to transmission plane angle and their phases are reversal. Then, the optimal solution formula of phase angle is deduced based on hard point of steering system and the computer program is compiled. The calculation result of the program is accurate by example verification. The conclusion and the program has theoretical and practical significance on system design of automobile steering and research on the transmission of other types universal joint. The cross-shaft universal joint can transmit torque while realizing angle changes. It has a simple structure, low wear and large transmission power, and the transmission angle between the main and driven shafts is relatively large. In the project, in order to realize constant velocity transmission, the double cross-shaft universal joint must meet the following three conditions [1]: ①The input shaft, the intermediate shaft and the output shaft should be in the same plane; ②The forks of the forks at both ends of the intermediate shaft In the same plane; ③ The angle between the input shaft and the intermediate shaft is equal to the angle between the intermediate shaft and the output shaft. Due to the compact layout of the car cockpit, the installation space of the steering column and steering gear is more restricted, and the steering drive shaft is difficult to meet the above three constant speed transmission conditions. Literature [2-3] studied the functional relationship between the input and output angular velocities of the double universal joint based on a single cross-axis universal joint; Literature [4-5] calculated the speed of the Hooke universal joint at any angle of rotation. Compared with the angle difference, it is concluded that the unequal speed ratio is both asymmetric and unbalanced; Literature [6] established the motion equation of the double Hooke universal joint, and deduced the middle of the double Hooke universal joint. The theoretical calculation formula of the relationship between the phase angle of the flange fork at both ends of the shaft and the transmission ratio. Adjusting the phase angle of the intermediate shaft is an effective measure to reduce the influence of speed fluctuations, but the existing literature does not systematically study the influence of the phase angle on the system speed fluctuations and give the optimal phase angle conclusion. How to select the optimal phase angle has become a key issue in the design of the steering system. Therefore, it is necessary to conduct an in-depth analysis of the influence of the phase angle of the double cross-axis universal joint steering drive shaft on the steering stability.