Abstract: Based on the Sentauus TCAD two-dimensional numerical simulation method, the FLR (Field Limit Ring) terminal single event bunout(SEB) mechanism of N-channel 250 V power VDMOS(Vertical Double-diffused Metal-oxide-Semiconductor) is deeply studied. On this basis, a reinforcement scheme for the termination buffer layer is proposed and its effectiveness is demonstrated through experiments. The most sensitive SEB position of the FLR terminal is between the main junction and FLR₁. After the incident of heavy ions, a large number of electron-hole pairs are generated, and collision ionization occurs under the acceleration of the leakage end electric field, resulting in extremely high transient current and local high heat triggering burnout. An optimization scheme for buffer epitaxial layer is proposed to improve the SEB performance of FLR terminals. Simulation results show that the buffer layer can weaken the impact ionization at SUB-EPI interface, reduce the peak current generated by heavy-ion incidence, and shorten the current recovery time, the SEB safety of FLR structure will improved more than 50%. The ¹¹⁸Ta ion experiment of the sample reinforced by the terminal buffer layer is verified. Compared with the common structure, the results show that the structure can effectively reduce the damage of heavy ions to the terminal region, and the IDSS leakage can be reduced by more than 4 orders of magnitude after irradiation.