The invention discloses a transmission gear locking mechanism which comprises a transmission with a transverse shift lever. A gear locking air cylinder is arranged on the side face of a transmission control shell, a gear locking air cylinder piston capable of limiting vertical movement of the transverse shift lever is arranged in the gear locking air cylinder, and a reset spring is arranged outside the gear locking air cylinder piston in a sleeving mode. The transverse shift lever is limited by the gear locking air cylinder piston in the gear locking air cylinder, so that the part change of the transmission gear locking mechanism is small, gear locking is reliable, and the transmission gear locking mechanism can be suitable for a direct gear and can be suitable for an overgear transmission; when the neutral gear of the transmission is located at different gears, a high reverse gear and a high climbing gear can be locked, high universality is achieved, the hand feeling is good, and high economic benefits and social benefits are achieved.