In this paper, a new analytical model for the surface electrical field distributions of double RESURF LDMOS is presented. Based on the 2-D Poisson solution, the model gives the influence on the surface electrical field in terms of the drain bias and structure parameters, such as the doping concentration, the depth and the position of the P-top region, the thickness and the doping concentration of the drift region and the substrate doping concentration; the dependence of breakdown voltage on the length of drift region is calculated. Further, an effectual way to gain the optimum high-voltage is also proposed. All analytical results are well verified by simulation results obtained by MEDICI and previous experimental data, showing the validity of the model presented here