In the backdrop of the growing energy crisis and drive for carbon neutrality, new energy vehicles have become a key area of interest. However, the complexity of the heat source distribution and the gradual increase in power density pose higher challenges to vehicle radiators. In this study, the heat transfer characteristics of the thermal management system of a range-extended hybrid electric vehicle were investigated. Furthermore, three cooling circuits and radiator arrangements were designed. The effects of different radiator arrangements on the heat transfer and flow characteristics were investigated. Additionally, power/pressure drop and power density values were proposed to evaluate the advantages and disadvantages of the radiator arrangement. The results reveal that the serial case (low-temperature radiator, air-to-air intercooler, and high-temperature radiator arranged in series) displayed a high power density of 2862 kW/m³ and a better power/pressure drop ratio of 0.15 kW/Pa. It exhibited the best overall heat transfer and flow performance considering the competition balance between pressure drop and heat dissipation. With an increase in the heat transfer capacity, the cooling flow rate generally exhibited an increasing trend.