Copper is relatively low cost and highly abundant compared with the well-studied noble metals such as gold and silver. However, the poor plasmonic and high susceptibility towards oxidation limit the study of its optical properties and applications as well. Herein, copper nanoparticles@polycarbonate (Cu@PC) composites were prepared by using a facile one-step solvothermal method. The Cu@PC composites have strong localized surface plasmon resonances (LSPR) due to that the PC shell can induce the particles to form many-particles system with different particle numbers, which not only lead to overlap and hybridize of the LSPR modes, but also shift the LSPR away from the interband transitions, and the PC layer also prevents the oxidation of Cu nanoparticles. The photothermal conversion efficiency of Cu@PC composites reaches 41.1% under 808 nm continuous wave (CW) laser irradiation which is higher than previously reported Cu nanomaterials that have been reported. Meanwhile, the composites also have high photothermal stability. Moreover, interfacial evaporator is prepared by assembling the Cu@PC composites on scouring sponge as light absorption layer which has > 92.8% absorption in entire solar spectrum range. Its seawater evaporation rate is 3.177 kg·m^–2·h^–1 with a E_evaporator/E_water of 5.2. The high evaporation rate interfacial evaporator with low cost, simple, and scalable approach shows great application value in the field of photothermal evaporation.