In the reliability-based analysis of mechanical joints, as each factor has different effect on the failure of mechanical joints, the effects of design parameters on reliability of the mechanical joints are determined with the reliability-based sensitivity analysis method. Furthermore, the reliability of mechanical joints can be effectively designed. As a matter of fact, if a change of a certain random parameter has great effect on the reliability of mechanical joints, the parameter must be strictly controlled in the course of design and manufacture to ensure reliability. Whereas if a change of a certain random parameter has unobvious effect on the reliability of mechanical joints, it can be treated as a deterministic value to reduce the complexity of the analysis. In this paper, based on the reliability design theory, the sensitivity analysis method and the fourth moment technique, the reliability sensitivity of the mechanical joints with arbitrary distribution parameters is extensively discussed and a numerical method for reliability sensitivity design is presented. The variation regularities of reliability sensitivity are obtained and the effects of design parameters on reliability of the mechanical joints are studied. The method presented in this paper provides reasonable and necessary reliability basis for the design, manufacture, use and evaluation of the mechanical joints.