Abstract: For the problem of insufficient longitudinal stability margin of a certain type of aircraft, an analysis method based on servo-structure modal coupling test and simulation is established. By comparing the sensor amplitude margin and response curve between the prototype and the verifier, it is preliminarily determined that the excessive response of the mounting structure is the main reason of the insufficient margin. The structural modal coupling simulation model is established through the control law simulation and the organism dynamics simulation, and the sensor response data is obtained by simulating the input signal of the test.Through the structural modal coupling simulation, it is proved that the double frequency phenomenon is the cause of the large response of the sensor installation structure, which leads to the lack of longitudinal stability margin. An adjustable frequency dynamic control device of sensor support system is proposed to effectively improve the stability margin of structural modal coupling test.