With the expansion of the New Energy Vehicles (NEVs) market, the demand for efficient thermal management systems is increasingly pressing. Topology optimization (TO) methodology is an important way to improve the performance of heat exchangers (HE) in many use scenarios. This study focuses on enhancing the design of air-side fins within plate-fin HE, which are vital components in NEVs thermal management systems. By employing the TO methodology, our research aims to enhance the hydraulic thermal performance of these HE. A pseudo-three-dimensional (3D) flow heat transfer model is used to improve the hydraulic thermal performance of HE by variable density method. An optimization function considering thermal resistance and dissipated power is proposed in this study. The investigation rigorously assesses the effects of different controlling parameters on the results of TO. Following the optimization, a 3D model of the optimized fin configuration is elaborated to analyze the localized fluid dynamics and thermal characteristics. Furthermore, the study evaluates the hydraulic thermal performance of the TO fins under diverse inlet velocities, revealing significant alterations in vortex patterns with velocity increments, which substantially impact heat transfer in the cooling channel. Under varying inlet velocities, the TO fin design exhibits the optimal hydraulic thermal characteristics, demonstrating an enhancement of 9.53% to 22.82% in the comprehensive performance factor (JF) compared to the conventional serrated fin. This research contributes innovative strategies for radiator optimization in thermal management systems of NEVs, fostering advancements in heat exchange efficiency and radiator lightweight.