For the purpose of describing the state transition process of the components and making accurate estimation to reliability of a multi-state system, the Wiener-Markov Model was used to compute the state probability, and the Universal Generating Function (UGF) method was used for reliability analysis of the multi-state system.A Wiener-MM and UGF based method was proposed for reliability evaluation of multi-state system.Verification and illustration were conducted with a certain type of radar power amplifier.The result shows that:this method decreases the effect of component failure threshold on system reliability estimation, and improves the accuracy of system reliability assessment.Thus a new method is provided for preventive repair of radar power amplifier, which is of great engineering application value.