A new method based on Geant4 and Technology Computer Aided Design (TCAD) is described to assess Single Event Upsets (SEU) sensitivity of SRAMs (Static RAM) in the design phase. SRAM's response to radiation and Heavy ion induced upsets was obtained by TCAD simulation instead of ground simulation experiments. Heavy-ion simulation data are fitted by Weibull Function. Then, the energy deposition of secondary particles produced by proton-silicon nuclear reaction was calculated using Geant4 which is based on Monte Carlo method. Finally, the proton-induced upsets can be achieved by combining with Weibull Function. Using this method, the heavy ion and proton-induced SEU rates in TSMC 0.18 μm SRAM were calculated, and the results are basically consistent with ground simulation experiments.