Power devices experience transient power surges under extreme operating conditions, leading to a sharp increase in junction temperature and posing significant challenges for thermal management. This paper designs and investigates a phase change heat sink based on the Pb-Sn-Bi-In low-melting-point alloy (LMPA) to address the thermal requirements of Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) power devices. Initially, a three-dimensional simulation model is established based on the actual dimensions of the device and the heat sink. Subsequently, the model’s continuity, momentum, and energy conservation equations are solved using numerical simulations to evaluate the effects of different power levels on chip junction temperature under both natural convection and forced air cooling modes. Simulation results across various operating conditions demonstrate that the phase change heat sink provides effective thermal management under natural convection with short-term power loads, while ensuring long-term stability and reliability of the chip junction temperature under forced cooling conditions.