Abstract: Research on a multiinput-multioutput(MIMO) aeroservoelastic (ASE) system was presented on the basis of modern control theory. The small gain theory and minimal singular value theory were applied to the control system which was equipped with the coupled roll and yaw loop. The stability scope of the aeroservoelastic system to resisted the model error was obtained. Both the methods gave the stability criteria respectively and according to the criteria the robust stability(or stability margin) could be defined. As an expression of turbulence the system uncertainty matrix was described. The above two methods result in the same result on the model of an ACT combat airplane.