Nanofluids, involving nanoparticles of the effective localized surface plasmon resonance (LSPR), exhibit the great enhancement in the light absorption and the heat transfer which activates many technological applications, especially covering solar thermal harvesting and photothermal therapy. In this paper, a numerical simulation method integrating classic electromagnetic wave optics, multiparticle group theory, heat transport and the Lambert-Beer's law was for the first time established via the finite element (FEM) method based on the COMSOL Multiphysics platform. The simulated optical and photothermal properties of both individual nanoparticle and its corresponding nanofluid revealed excellent LSPR-effectiveness in a variety of core-shell structured nano particles, and the simulation was verified by direct theoretical solution based on the Mie scattering and openly-sourced experiment results. Among of them, a core-shell structure nanoparticle with the copper core in diameter of 90 nm coated with 5 nm thickness graphene was found to be the most economical and effective in the photothermal performance within the solar spectrum. The photothermal performances of two-particle and eight particle models revealed that highly dispersed nanoparticles were more conducive to the overall temperature rise. The synergistic effect of LSPR-effective nanoparticles and far-IR absorption effective base solution enhanced the photothermal performance of nanofluids corresponding to copper/graphene core-shell nanoparticles. This research provided an efficient method to screen advanced LSRP-effective nanoparticle candidates and optimize the photothermal conversion of nanofluids for solar energy harvest.