Radiative cooling, a unique and uncommon passive cooling method for devices operating outdoors, has recently been demonstrated to be effective for photovoltaic thermal management. In this work, we investigate the effect of radiative cooling as a complement to existing passive cooling methods like convective cooling in a related system with much higher heat loads: a high-concentration photovoltaic (HCPV) system. A feasible radiative cooler design addressing the thermal management challenges here is proposed. It consists of low-iron soda-lime glass with a porous layer on top as an antireflection coating and a diamond layer as heat spreader. It is found that the proposed structure has strong mid-IR emittance as well as high solar transmission, allowing radiative cooling under direct sunlight and low loss in the concentrated solar irradiance. A systematic simulation with realistic considerations is then performed. Compared with a conventional copper cooler, the lowest temperature reached by the proposed radiative cooler is 14 K lower. Furthermore, less area of the proposed cooler is needed to reach a standard target temperature (333.15 K) for steady-state operation under high concentrations for the crystalline silicon PV module. In order to compare the coolers quantitatively, a figure of merit – cooling power per weight - is introduced. At the target temperature, the proposed cooler is determined to have a cooling power per weight of 75 W/kg, around 3.7 times higher than that of the conventional copper cooler.