Abstract: Using silkworm cocoons as raw material, four types of sericin with different molecular weights were successfully prepared via hot water extraction by adjusting the extraction time. With prolonged extraction time, the molecular weight of the obtained sericin gradually decreased. Using this series of sericin proteins as biomolecular templates, copper sulfide nanoparticles (CuS@Ser NPs) were synthesized, and their photothermal properties were systematically investigated. The results indicated that sericin with different molecular weights could all serve as effective templates for the synthesis of CuS@Ser NPs; moreover, the smaller the molecular weight of sericin, the smaller and more uniformly distributed the resulting CuS@Ser NPs particles. Photothermal performance analysis revealed a significant correlation between the photothermal behavior of CuS@Ser NPs and the molecular weight of the template sericin: the smaller the sericin molecular weight, the better the photothermal performance of CuS@Ser NPs, manifested as a higher final equilibrium temperature, faster heating rate, and greater photothermal conversion efficiency. This study provides both theoretical insights and experimental support for optimizing the photothermal performance of copper sulfide materials by regulating sericin molecular weight, and provides a valuable reference for expanding the application of sericin-based composite materials in the field of photothermal conversion.