Abstract: Direct imaging of exoplanets can be achieved with the coronagraph module of the China Space Station Telescope (CSST). However, its motion module will generate micro-vibration disturbances during orbital operation, resulting in a decrease in the image quality of the coronagraph. In this paper, the nonlinear modeling analysis on the commonly-used vibration isolation system for spacecraft loads was carried out. The nonlinear expansion of Kelvin-Voigt model for viscoelastic materials was conducted. The constitutive equation was solved using the third-order harmonic balance method. The vibration equation of nonlinear Kelvin-Voigt model was constructed in differential forms, providing a theoretical foundation for analogous design assumptions and engineering applications. The model analysis show that the expanded nonlinear solid model effectively supplements the design theory of nonlinear elastic micro-vibration isolation system, which is conductive to enhance the engineering data accuracy.