Abstract: The dynamic equation of a rigid-flexible coupling nonlinear system is derived based on Lagrange Equation.The effect of satellite's relative three-dimensional motion on orbit coordinate,flexible vibration of antenna's surpporting beam caused by satellite motions,the nonlinear angle displacement resulted from the flexible of GDA and scanning motions of the antenna are considered.This dynamic equation shows the coupling motion and dynamic behaviors more completely.Thus some simplified dynamic equations on different conditions are derived.So,when the original system's vibration is analysed,just the corresponding result is obtained.The method is also suitable for analysing dynamic behaviors of the general.