An improved 4H-SiC superjunction UMOS device is proposed. Splitting p-pillars and thick bottom oxide are adopted in this structure. P-pillars can form a superjunction structure with N-drift layer, which reduces the specific on-resistance while maintaining a high breakdown voltage. Thickening the bottom oxide will reduce the gate-drain capacitance. Combined with the actual MOSFET fabrication process, the ion implantation condition is given through SRIM simulations, and the device parameters are optimized through Silvaco TCAD simulations. Finally, a superjunction UMOS device with a breakdown voltage of 1035 V and a specific on-resistance of 0.886 mΩ.cm_² is obtained.