The invention discloses a split torque transmission speed reduction device for a helicopter. Each engine input shaft is connected with a power input shaft through an overrunning clutch; a straight toothed spur gear on each power input shaft is engaged with two face gears installed in the opposite directions to achieve the power shunting, first-stage speed reduction and the reversing functions. The face gears are connected with the corresponding straight toothed spur gears through corresponding duplicate gear shafts and transmit power uniformly to the four straight toothed spur gears to achieve the second torque split and second-stage speed reduction functions. Power is transmitted to four herringbone toothed spur gears by the four straight toothed spur gears through the four duplicate gear shafts, and the four herringbone toothed spur gears are engaged with a fifth herringbone toothed spur gear correspondingly to achieve the second-stage speed reduction and parallel operation functions. The split torque transmission speed reduction device adopts multistage gear transmission, and is high in transmission ratio and low in vibration noise. Power is shunted through multiple branches. Torque transmitted by the gears in all the branches is reduced. The bearing capacity of the gears is improved. The fatigue life of the gears is prolonged. Accordingly, a transmission system has higher survivability.