In this paper, the low melting point metal phase change material (PCM) heat sink for coping with ultra-high thermal shock (10² W/cm²) is developed and optimized theoretically and numerically. Gallium is selected as the best PCM candidate from the point of view of thermal performance based on an approximate theoretical analysis, and gallium based PCM heat sink with internal copper fin is configured. Different fin structures, namely plate fin, crossed fin and pin fin are investigated and compared; the effects of fin number, fin width and base thickness are parametrically studied; the influence of the structural material is briefly discussed. For arbitrarily given heating condition, the optimal geometric configuration of the heat sink is suggested and corresponding thermal performance is provided. The proposed low melting point metal PCM heat sink can cope with very large thermal shock like 100 W/cm² (1 s) with maximum device temperature of 46 °C, under the ambient temperature of 25 °C, which is extremely difficult to deal with otherwise by conventional PCMs. The conclusions drawn in this paper can serve as valuable reference for thermal design and analysis of PCM heat sink against ultra-high thermal shock.