Abstract: The parallel system is widely used in the civil aircraft in order to enhance the reliability.The redundancy units have the same physical characteristic and circumstance,and the failures are found inconveniently,so the imperfect crossed inspection is proposed firstly.Then the failure dependency and modes between these two units are analyzed,and the renewal probability of the parallel system is studied.Furthermore,the inspection interval and number are chosen as optimized variables,and an optimal maintenance model is developed to minimize the maintenance cost rate,and the model is subject to the reliability.Finally,an example is illustrated for case study,and the result proves the economicness and effectiveness of the model.