In this article, an ultrahigh repetition frequency sub-nanosecond pulse generator based on vanadium-compensated semi-insulating (VCSI) 4H-SiC photoconductive semiconductor switch (PCSS) under high bias electric field is presented. In this work, the 0.8 mm-thick 4H-SiC PCSS was found to work with electric fields up to 200 kV/cm and power capacity of 176 kW. The VCSI 4H-SiC PCSS response test was performed with a 1 MHz, 1030 nm laser cluster driver with configurable optical pulsewidth and current viewing resistor (CVR) high frequency response circuit. In the experiment, the laser pulsewidth was varied between 300 and 500 ps, and 1.5 ns. The system generated a 1 MHz repetition frequency pulse with a minimum half-width of 365 ps and less than 100 ps of jitter. The results revealed higher response performance for single/continuous pulse triggered light of various pulsewidths. Due to the low absorption efficiency of 4H-SiC for non-intrinsic light, an axial optical reflection trap structure was utilized to boost the light absorption efficiency of PCSS. The proposed structure significantly improved the light absorption performance of PCSS devices. The photocurrent increase ratio of roughly 115% was obtained. This study presents the first results of MHz re-frequency high-power switching in bulk semi-insulating 4H-SiC.