Abstract: In this paper, multi-step sidewall oxide layers are applied to split-gate trench MOSFET (Split-Gate Trench MOSFET, SGT structure), and the improved structure is called a multi-step sidewall oxide layers split-gate trench MOSFET (Multi-Step Sidewall Oxides Split-Gate Trench MOSFET, MSO structure). Then the device structure and manufacturing process of the MSO structure are introduced. The epitaxial layer doping concentration, the thickness of the top and bottom of the sidewall oxide layer of the MSO structure are optimized by TCAD simulation software. Finally, the simulation results show that the MSO structure with the breakdown voltage is 126 V, a specific on-resistance of 30.76 mΩ·mm² and a specific gate drain charge of 0.351n C·mm^-2. At approximately the same breakdown voltage, specific on-resistance and specific gate-drain charge of the MSO structure are lower than those of the conventional SGT structure. These two parameters show that the figure of merit (FOM=Q_{gd, sp}×R_onA) of the device is decreased by 39.6%.