DC solid-state circuit breakers (SSCBs) based on SiC MOSFET have attracted much attention owing to their capability of quickly breaking the fault currents. However, the failure mechanism of SiC MOSFET is more complex than those of traditional power semiconductor devices. SiC MOSFET has special gate failure and aging characteristics, but the relevant characteristics and mechanism are not clear, which brings great challenges to the failure and aging discrimination of SSCBs. A 400 V DC SSCB based on SiC MOSFET was taken as the research object, and the single and repetitive hard switching characteristics of the device under this working condition were investigated. It was verified that gate failure may occur in SiC MOSFET during the process of single or repetitive actions, and a new aging characteristic of gate voltage reduction appeared. Based on simulation tools, the temperature and thermal stress distribution inside the device during the process were obtained. It was found that the temperature rise near the gate structure of the device was close to 1 000 K, and a thermal stress of 1.4 GPa would be generated. Through the opening, slicing and SEM scanning of failed devices, the cracks in the gate oxide layer were confirmed, and the mechanism of gate failure and aging caused by gate structure cracking due to thermal stress was verified.