This paper presents a low melting point Bi–Pb–Sn–Cd alloy as the phase change material (PCM) for a thermal management system. Compared with an organic PCM (stearic acid (SA)), which has a similar melting point of ∼69 °C, the thermophysical properties of the two PCMs were characterized, which revealed that the low melting point alloy (LMPA) had much larger thermal conductivity (∼125.22 W/mK) and volumetric latent heat (∼365.5 MJ/m3). The analysis results of a 270-day compatibility experiment using scanning electron microscopy (SEM) and the energy dispersive spectrum (EDS) showed that the LMPA had good compatibility with the aluminium alloy and copper. With the same dimensions, the LMPA-based heat sink outperforms the SA and copper foam/SA composite based heat sinks at various power input levels, which could effectively reduce the temperature of the heater and extend the effective managed time at least 1.5 times. The LMPA-based heat sink could also discharge heat more efficiently, which is preferred for cyclic operation.