Objective: The internal flow channel design and oil immersion depth of gearboxes play a crucial role in determining the lubrication effectiveness of gears and the temperature rise within the gearbox. These effects intensify as train speeds increase. This study focuses on a specific high-speed rail aluminum alloy gearbox, using Simcenter STAR-CCM+ (hereinafter referred to as Star CCM+, a multi-physics simulation software) simulation software to develop a thermal-fluid-solid coupling simulation and analysis model. By integrating the simulation results with bench test data, this study aims to investigate the internal flow field and temperature field of the gearbox. The effects of various factors, including rotational speed, oil immersion depth, and steering direction, on the flow and temperature distributions within the gearbox are examined, providing insights into optimal operating conditions and potential design improvements. Methods: To analyze the performance of the gearbox, parameterized simulation analyses were performed considering different rotational speeds, oil immersion depths, and steering directions. The distribution of the internal flow and temperature fields under these varying conditions was studied. The analysis also focused on the mass flow rate and temperature field of each flow channel. This comprehensive approach allowed for a detailed evaluation of the lubrication performance of the gearbox. The Star CCM+ simulation model was calibrated using experimental data from a 1∶1 test bench, where temperature measurements were taken at various points within the gearbox. These measurements were compared with the simulation results to ensure the accuracy and reliability of the simulation model. The study also incorporated detailed thermal conditions, including gear frictional power losses, bearing power losses, and forced convection heat transfer, to represent the true working conditions of the gearbox under different operational scenarios. Results: The simulation results showed that the lubrication and temperature control effects of the gearbox were most effective when the internal oil immersion depth was between 1.75 and 2.00 times the tooth height. It was found that insufficient lubrication occurred on the upper and right sides of the gearbox, highlighting areas that require design improvements. Additionally, the research revealed that increasing the oil immersion depth improves the flow and distribution of lubrication oil within the gearbox. However, a deeper oil immersion beyond the optimal range increases churning losses and heat generation. By adjusting the flow channel configuration and improving the number and distribution of the internal flow paths, the optimized design reduced the temperatures in critical areas, including the bearing and meshing zones, by approximately 5℃. This improvement was achieved by increasing the oil flow to the gears and bearings while enhancing the cooling effect on the gearbox walls. Conclusions: This study demonstrates that a proper oil immersion depth is critical for maintaining effective lubrication and temperature control in high-speed rail aluminum alloy gearboxes. The results highlight that there is an optimal oil immersion depth range (1.75-2.00 times the tooth height) that ensures sufficient lubrication and effective cooling. Furthermore, the study reveals that there are areas within the gearbox, particularly on the upper and right sides, where lubrication is insufficient, suggesting that the design of the flow channels in these regions can be improved. The proposed modifications, such as the addition of more flow channels and optimizing their distribution, provide a substantial enhancement in the lubrication and cooling efficiency of the gearbox. These modifications result in a notable temperature reduction of approximately 5℃ in key areas, thereby demonstrating the effectiveness of the flow channel optimization strategy. This research provides insights into future gearbox design, particularly in optimizing lubrication systems and minimizing temperature rise to ensure the reliable operation of the system at high speeds.