To accurately reflect the switching characteristics of GaN High Electron Mobility Transistor (GaN HEMT), this article proposes a fast and accurate Spice circuit simulation model of GaN HEMT. The model mainly imports the saturation current modulation factor θ(Vgs) and focuses on the modeling accuracy of the nonlinear capacitance Cgd, which would affect the switching process. In addition, to address the present problems of relying on commercial fitting software with high copyright fees for parameter extraction and extracting model parameters very intricately by traditional modeling methods, a combined optimization method is proposed to extract the model parameters for modeling. The combined algorithm is based on the Genetic algorithm of global optimization and the Levenberg–Marquardt algorithm of the local optimum and it has the advantages of the global optimal solution and fast convergence. The proposed modeling method is verified to be fast and accurate by building a Double-Pulse Test experimental platform and comparing the simulated and experimental waveforms of the dynamic and static characteristics of GaN HEMT.