To solve the problem of high risk and high cost in direct flight test of full-size aircraft, the sub-scale model is used to carry out relevant tests. Based on the similarity criterion of wind tunnel test and the 6 degrees of freedom motion equation of aircraft, the similarity theorem is used to study the similarity criterion of motion and the similarity criterion of flight control system for the sub-scale model flight test. Taking a fixed-wing aircraft as an example, the open loop and closed loop 6-DOF simulation of the sub-scale model and the full-size aircraft are carried out, and the similarity criteria derived are verified. The results show that the stability and control characteristics of the original aircraft can be accurately reflected by the test results.