For a fairly long time， it is assumed in power system reliability studies that the state transition functions should follow exponential distribution while in engineering practice， but the preventive maintenance interval is typically non-exponential. Therefore， the assumption is bound to seriously affect the credibility of analysis results. For this reason， this paper proceeds from the fundamental theory of reliability in mathematics， and with the relay protection system as an example and taking into account the preventive maintenance， a model for system reliability with multi-outage modes and its analytical solution based on the semi-Markov process are studied and developed. Thus the limitation of the frequency-duration method requiring exponential distribution transition function is broken through. The advantages of the proposed method are reflected in high precision and high speed. With the actual relay protection operation statistical data as the basic data on reliability， by comparing with numerical results based on sequential Monte Carlo simulation， the proposed reliability model and its analytical solution are fully proved of scientific significance and engineering application value in power system reliability application research.