To reduce the operating temperature of the high-speed double-helical gear transmission system, the oil-gas two-phase fluid dynamics model of the gear transmission system was established based on the RNG k-ε turbulence model. The internal flow field and convective heat transfer characteristics of the gearbox with different diversion structures were simulated. The time-varying friction coefficient and heat flux of the tooth surface were calculated by the elastohydrodynamic lubrication (EHL) theory. The heat conduction finite element model of the gear pair was built, and the influence of the position of the diversion hood and diversion plate on the temperature field of the gear pair was analyzed. The results show that the motion path of the lubricating oil on the tooth surface was an inclined straight line pointing to both ends of the gear. When the diversion hood and diversion plate are close to the tooth top of the gear, the amount of lubricating oil on the tooth surface increases, and the convective heat transfer coefficient (CHTC) also increases. The temperature at the top and root of the tooth is higher, the temperature at the pitch line is the lowest, and the highest temperature appears at the top of the tooth. The maximum temperature rise of the tooth surface decreases as the diversion hood and diversion plate move toward the top of the tooth.