Based on the traditional Archard wear theory, a novel wear model of ball screws was established by considering the actual contact condition between the ball and the raceway. According to the stress strength interference theory and the improved first-order second-moment method, the model of the reliability and sensitivity of the wear resistance of ball screws was proposed. The numerical example of both the reliability and sensitivity of a typical miniature ball screw (1004) in a small satellite was presented. The unit sensitivity was defined and proposed to better indicate the influence of different parameters on the wear resistance of ball screws, based on which the 1004 ball screw was redesigned and the wear resistance reliability of the ball screw was improved by 21%.