Aiming at effectively diagnosis for air condition system fault of the special vehicle and correctly finding out fault reason in time, research fault diagnosis method of air condition system of vehicle based on fuzzy diagnosis theory. Analyze the air condition system fault mode, establish fault fuzzy math model, ascertain the fuzzy relation of fault symptoms and fault reason, and determine the comprehensive fuzzy membership based on experience maintenance and expert diagnosis. According to max membership principle, determine the max membership of fault symptoms to fault reason, then find out the reason for air condition system. The research results show that the method can correctly and reliably find out the fault location and effectively shorten the maintenance time.