Abstract: The allowable ambient temperature is an important evaluation index of the cooling system of heavy duty vehicles, directly affecting vehicle power performance, handling performance, life-cycle economy, and the reliability of key components. Among these factors, the fan type has a significant impact on cooling system performance. Taking a heavy duty vehicle as the research object, a mathematical model of the under-hood flow field is established. ANSA and STAR-CCM+ software are used to numerically simulate and analyze the flow field performance of an open fan and a ring fan, respectively. The analysis of model characteristics and simulation results shows that, under the same dimensional conditions, the ring fan provides a larger air volume and better heat dissipation performance. Finally, a full-vehicle thermal balance test is conducted on a high-power tractor. The experimental results are in good agreement with the simulation results, verifying the accuracy of the model and providing an effective reference for fan matching design in heavy duty vehicle cooling systems.