Maintenance plays an important role in guaranteeing facilities' safety operation, improving facilities' quality, keeping and delaying engineering system lifetime as long as possible. Reasonable and effective PM measures are the strong guarantee for equipment's well operation. Based on PM, this paper tries to overcome the shortages of conventional regular maintenance, and puts forward an optimal policy which is more realistic and closer to the real maintenance situation. In this optimal model, when system's reliability reaches the threshold R-0, breakdown for PM is needed. In this paper, reliability threshold is regarded as a constraint, and the goal is to minimize the expected long-run total cost rate. At last, a numerical case is used to verify that the optimal model has more practical significance. This paper provides well ideas and methods for well equipment running.