The mechanical equipment faults result from parts failure in the period of service time. Due to reassembly and maintenance, the reliability model for mechanical equipment is broken, so it is necessary to research and estimate the safety reliability of mechanical system. Based on the timetofailure density function of parts, the mechanical system reliability model is constructed to track the change course of age structure of part population for the mechanical systems that are reassembled and maintained. By means of simulation of the system reliability model, concerned parameters with mechanical systems service life are defined.The paper discusses how the timetofailure density function hase influence on the service life for mechanical systems undergoing reassembly and maintenance. It is significant for estimating reliability and failure rate of systems and establishing reasonable maintenance policies.