Based on the three-dimensional model of Rzeppa constant velocity joint, the formula of circumferential clearance angle at any swing angle and rotating angle was deduced, and the optimal assembly position of driving shaft assembly was analyzed by using MATLAB software. The assembly requirements were proposed based on optimal circumferential clearance fluctuations. The results show that the circumferential clearance at both ends of drive shaft varies with the swing angle between the input and output. The value and fluctuations of the angle can be reduced effectively in an appropriate assembly position, and the vibration and noise can be weakened during travelling.