To investigate the effects of lubricating oil temperature and viscosity on power loss and key component temperature of the duplex reduction gearbox for a certain type of tandem unmanned helicopter, and optimize the oil distribution of the lubrication system, this study first constructed a heat generation-heat transfer-lubrication coupled AMESim simulation model based on the thermal network method. Subsequently, the influence laws of lubricating oil temperature and viscosity on internal power loss and temperature distribution of the reducer were analyzed. On this basis, with the minimum average temperature of components as the objective, the oil distribution of the lubrication system was optimized using a genetic algorithm, and the optimal lubricating oil injection hole diameter was solved by integrating lubricating pipeline parameters. The results indicate that: When lubricating oil temperature rises, the viscous resistance of rotating parts decreases and convective heat transfer is enhanced, while tooth surface sliding friction loss and total power loss increase. Increasing lubricating oil viscosity can reduce the overall power loss and radiator outlet temperature, but the temperatures of both bearings and gear meshing areas rise—for bearings, this is due to increased oil churning loss and weakened heat transfer; for gear meshing areas, the effect of weakened heat transfer outweighs the reduction in meshing loss. After oil distribution optimization, the average temperature of components decreases by 1.89% compared with the original oil supply scheme based on heat generation proportion. This study provides a theoretical reference for the optimal design of lubrication and heat dissipation systems for such gearbox.