In order to improve the mission completion capability of Line Replaceable Unit (LRU) system, and reduce the maintenance resource consumption, a maintenance decision method for multi-state system based on Non-Homogeneous Continuous Time Markov Model(NHCTMM) was provided.NHCTMM was used to describe the degradation process of components, and the universal generating function was used to analyze the reliability of the system.Based upon the imperfect maintenance decision model, an imperfect preventive maintenance decision method was proposed, which ensured that the system could obtain maximum benefit-cost ratio in the limited service period.Verification was conducted to a radar power amplifier LRU system, and the result showed that this method has a strong versatility and engineering application value.