Abstract: Oil bath lubrication is a significant lubrication and heat dissipation method in gear transmission systems, widely employed in applications such as electric vehicle reducers and helicopter tail gearboxes. The effectiveness of oil bath lubrication directly impacts the operational temperature, stress, and strain of gear transmission systems, with inadequate lubrication leading to risks of gear surface wear, adhesive wear, and pitting. This study focused on the standard FZG gear test gearbox and constructs a numerical simulation model coupling heat and fluid flow for gear transmission systems with oil bath lubrication using a thermal-fluid sequential coupling approach. In flow field analysis, the volume of fluid (VOF) two-phase flow model and renormalization group (RNG) k-ε turbulence model were selected to ensure analysis accuracy and computational speed. In temperature field analysis, user-defined functions (UDF) were utilized to incorporate frictional heat flux and convective heat transfer coefficients at different gear surface locations, ensuring temperature analysis accuracy. The study revealed fluid field and component temperature variations in the gearbox under different FZG load levels and rotational speeds. Simulation analysis showed that under steady-state conditions, an oil stirring cavity formed around the pinion in oil bath lubrication, primarily relying on oil agitation from the tooth crest of the wheel for lubrication. Due to significant sliding speeds at the tooth crest of the pinion and the tooth root of the wheel, high frictional heat flux resulted in noticeable temperature rises, with the pinion exhibiting higher temperatures than the wheel, reaching 119.8 ℃ and 110.0 ℃, respectively, posing considerable risks of adhesive wear failure. As load levels increase, the temperature difference between the gear surface and the oil increased, enhancing the oil's heat dissipation capability. With load levels rising from FZG level 5 to level 9, the pinion consistently exhibited the highest temperature in the system, reaching a maximum temperature increased from 119.8 ℃ to 178.2 ℃. As rotational speed increased, the oil agitation torque exponentially increased, with oil agitation power loss at an input rotational speed of 21 750 r/min accounting for 79% of the total gearbox system power loss. Component temperatures gradually rose with increasing rotational speed, with significant temperature rose observed in bearings at maximum speed, becoming potential failure points in the transmission system. Experimental validation using a wireless temperature testing system for FZG gear and gearbox temperature tests revealed that the model prediction error compared to experimental results was within 6%, preliminarily verifying the accuracy of the FZG gearbox thermal-fluid coupling model. These findings provided theoretical support for temperature estimation and control in gear transmission systems.