The preload torque is an important factor that affects the reliability of the ball screw. At present, the calculation of preload torque is mainly based on the empirical formula. There is a large error between the theoretical calculation value and the actual preload torque value. To calculate the preload torque of the ball screw accurately, based on the contact deformation and load distribution theory, the influence of key machining dimensional errors on the preload torque is analyzed. The relationship model among pitch diameter error, lead error and preload torque of ball screw is formulated. The lead error and pitch diameter of different types of ball screws are measured by the ball screw lead error and the profile detection test bench. The theoretical value of preload torque of ball screw is calculated through the lead error and pitch diameter error. The actual value of preload torque is measured by the preload torque test-bed of ball screw. The measurement results show that the relative error between the theoretical calculation value of preload torque and the test value is 0. 65% ~ 12. 18% , which is less than 15. 07% ~ 45. 57% of the existing models. The consistency between the theoretical model and the actual friction torque is accurate. The correctness of the theoretical model is verified.