The objective of this research was to supply theoretical bases for design and optimization of stiffness and damping parameters of active suspension system based on LQG control. Taking linear and nonlinear half car models with 4-degrees of freedom (DOF) as research examples respectively, influences of the above two parameters on active suspension system were analyzed through theoretical derivation and numerical simulation. Theoretical derivation about the linear model showed that the Ricatti Equation’s solution of the active suspension system based on LQG had nothing to do with stiffness and damping parameters. Result came as that the time domain response of the active suspension system had nothing to do with stiffness and damping parameters. Numerical simulation about the linear model revealed that each integral force made up with stiffness force, damping one and active control one were independent of the other and invariable when stiffness and damping parameters were changed, and the global damping characteristics of the active control obviously became negative from positive with its parallel damping parameter increasing. As for the nonlinear car model in general, three steps, including linearizing the control system, designing LQG controller for the linearized control system, and de-linearizing in control implement, were executed to achieve the LQG controller for the nonlinear active suspension system. The design process showed that the integral forces were not influenced by stiffness and damping parameters, because their force actions were counteracted in the above linearizing and de-linearizing steps. Thus, the above results about the linear active suspension system were also true when the active suspension system was with nonlinear stiffness and damping characteristics.