The fatigue life prediction and reliability analysis of dynamic systems under random excitement are important topics in modern engineering design. However, the stress power spectral density and the formulation of fatigue life prediction of a component in a dynamic system must be known when one predicts its fatigue life. A method of simulating computation for the fatigue life of a dynamic structure is presented. The method is based on the concept of unit load stress matrix. According to it, the relationship between the stress power spectral density of a structure in a system and the response spectra of the system can be established. Based on this, the formulation of the fatigue life prediction which is decided by the random stress process is obtained. With this method, the stress power spectral density simulation and the reliability fatigue life prediction for the CW-200 type vehicle truck are made. The method is suitable not only for the running dynamic structure but also for newly designed dynamic structures especially.