For the characteristics of capacitor’s stochastic deterioration, the research studied the scheme design and strategy optimization problem of its preventive maintenance based on semi-Markov process. Taking the inspecting time as optimization variable and considering the threshold of preventive maintenance times, the research built the long-run expense rate optimizing model of the capacitor based on the Gaussian-Poisson process model of its stochastic deterioration. The analysis shows that for the capacitors preventive maintenance is superior to posterior replacement in the saving of long-run expense rate.