In this paper, the variation rules of strength, load, reliability and failure rate of mechanical components are studied with time, and a state equation model for dynamic reliability of mechanical components is established under random load. Meanwhile, reliability index is obtained by using Second moment method and perturbation method. Based on the reliability design theory and sensitivity analysis method, torsion-bar is taken as an example, and the dynamic reliability sensitivity of torsion-bar is extensively discussed and a computing method is presented for dynamic reliability sensitivity design. The variation rules of dynamic reliability sensitivity are obtained and the effects of design parameters on reliability of torsion-bar are studied. As a result, the proposed method provides theoretical basis for reliability design of torsion bar.