MMEL (master minimum equipment list) projects approved for use by domestic and foreign civil aircraft, especially those with high recognition among users, can be considered as PMMEL projects of alternative target aircraft. Taking the air conditioning system of high frequency as an example, the similarity algorithm of aircraft system was studied. When analyzing the PMMEL requirements of the target aircraft, the similarity of the system function and structure of the equipment or project was considered, and the similarity criterion judgment technology was formulated. By using this technology, the PMMEL project list of the target aircraft can be extracted, which makes the user demand analysis of civil aircraft manufacturers comparable. The similarity calculation was based on Euclidean distance and weight. Taking the air-conditioning distribution system with high utilization rate of MEL (minimum equipment list) items in the route as an example, the feasibility of similarity algorithm and similar case extraction technology was verified, and it is concluded that the method of determining the PMMEL alternative suggestion list can be considered according to similar models. This paper summarizes some similar parts of air conditioning systems of A320 and Boeing 737 aircraft, which provides a theoretical basis for compiling and establishing a database of similar aircraft.