In order to reduce direct maintenance costs of civil aircraft, which is an important way to improve aircraft economics and enhance market competitiveness. Based on the background that the maintenance cost is largely determined in the development stage of civil aircraft, by constructing a hierarchical diagram of the design parameter structure that reflects the cost characteristics of civil aircraft, the TOPSIS-Grey correlation method model is established to calculate the relative similarity of similar aircraft and to carry out the direct maintenance cost allocation of civil aircraft. The direct maintenance cost data of typical systems of similar aircraft types is input into the allocation model for verification. The results show that the model in the article can predict the system-level direct maintenance cost of civil aircraft well, and the calculation result can meet the engineering requirements, which can provide a reference for predicting the direct maintenance cost of the newly developed aircraft.