The article deeply analyzes the design of sustained motor to solve the problem of insufficient thrust which occurred in a test on high altitude localities. As the article points out, the press reduced as the altitude raised, which cause the changed of the matching between the nozzle outlet pressure and atmospheric pressure and the change of the gas flow field. Digital simulation and experiment proved the validity of the theory. Besides, the article also proposes improved methods. By adjusting the expansion ratio of engine nozzle, we improve the difference of the engine nozzle outlet pressure of and the atmospheric pressure, and ensure the performance of the cruising engine on the plateau. As prove in test, the improve scheme is simple and practical.