In order to meet the design requirement of the heatsink for auxiliary power inverter and achieve the best system matching, taking one auxiliary power inverter heatsink as study object, the heatsink thermal resistance affected by geometry parameters was analyzed, and a high power density heatsink was designed and optimized based on theoretical calculation, numerical simulation was applied to develop an analytical model for the heatsink with CFD software, which optimized the heatsink parameters. Experiment proved that simulation results were 5%~8% in error, and the simulation methods was reliability for reducing development period and design cost.