The invention discloses a simulation system and method for the withstand voltage test of an extra-high voltage GIS (Gas Insulated Switchgear). The system comprises an initial parameter receiving module, a discrete component model building module, a calculation model building module and an analysis module, wherein the initial parameter receiving module is used for receiving initial parameters of discrete components and setting initial parameters of the discrete components in the simulation system; the discrete component model building module is used for inputting the initial parameters of all discrete components, which are received by the initial parameter receiving module to build each discrete component model; the calculation model building module is used for building calculation models by using all discrete component models built by the discrete component model building module; the analysis module is used for determining the actual withstand voltage test parameters of the GIS according to simulation results. The system and method provided by the invention can overcome the defects of low reliability, poor security and large economic loss and the like in the prior art, and have the advantages of high reliability, good security and small economic loss.