One of the research hotspots for Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors (SiC MOSFETs) is how to reduce the High-Frequency Figure of Merit (HF-FOM). This paper presents a theoretical derivation combined with TCAD simulation technology to analyze the influence of field plate isolation dielectric materials on device performance. Based on this analysis, we propose a split-gate MOSFET structure (HK-MOSFET) that connects the source field plate with a High-K field plate isolation dielectric. Compared to the HK-MOSFET(SiO2) using silicon oxide as the field plate dielectric and the conventional SG-MOSFET, the gate-drain capacitance (CGD) of the HK-MOSFET(HfO2) using hafnium oxide as the dielectric is reduced by 34.0% and 71.7%, respectively, while the gate-drain charge (QGD) is reduced by 24.5% and 58.9%, respectively. The high-frequency figures of merit HF-FOM1(C_GD×R_onsp) and HF-FOM2(Q_GD×R_onsp) decrease by 71.4% and 60.1% compared to the SG-MOSFET. The results indicate that utilizing High-K dielectrics is an effective approach for designing fast-switching SiC MOSFET devices.