An analytical model for the surface electrical field distributions of double RESURF devices with Gaussian-doped P-top region is presented. Based on solvisng the 2-D Poisson equation, the model gives the closed form solutions of the surface potential and electrical field distribution as functions of the structure parameters and drain bias. The dependence of breakdown voltage on drift region length and thickness is calculated. An effectual way to get the optimum high-voltage devices is also proposed. All analytical results are well verified by simulation results obtained by MEDICI and previous experimental data, showing the validity of the presented model.