According to the requirements of altiplano airport and the prohibition of a fighter to down landing-gear above 4000 meters, this paper makes research on the problems of the safe altitude of down landing-gear. The secure model of man-aircraft closed-loop system is given by means of mapkob model .Aviation risk of down landing-gear above 4000 meters is valued basing on calculation and analysis, special secure control methods are proposed .The theoretical results accord with the simulating flight and the real test.