The invention discloses a speed reducer layout structure used for a range-extending type electric vehicle, and belongs to the technical field of electric vehicle speed reducers. The speed reducer layout structure comprises a power generation transmission mechanism, a drive transmission mechanism and a differential mechanism, wherein the power generation transmission mechanism comprises a power generation input shaft, an idle wheel shaft and a power generation output shaft; the drive transmission mechanism comprises a drive input shaft and a drive output shaft; power output by an engine is transmitted into the power generation transmission mechanism through dual mass flying wheels, and rotation speed is increased by the power generation transmission mechanism through a gear wheel of each axle, so that a power generator is driven to generate power in a high-efficiency area, and therefore, electric power is provided to a whole vehicle power battery and a drive motor; power generated by the drive motor is transmitted to the differential mechanism through secondary speed reduction of the drive transmission mechanism, and the differential mechanism is used for transmitting power to wheels to drive the whole vehicle to travel. The power transmission mechanism is more compact and simpler in structure, is convenient in assembly, and effectively overcomes the defects that an existing electric vehicle is low in endurance mileage and is high in cost due a too great power battery.