The invention discloses an auxiliary vehicle running transmission device based on a retarder, and belongs to the field of vehicle transmission systems. The auxiliary vehicle running transmission device comprises a power input shaft, a power output shaft, a differential motion wheel system and the retarder. The differential motion wheel system comprises a sun wheel, a planet wheel, a planetary gear frame and a gear ring. The power input shaft is coaxially connected with the sun wheel. The sun wheel is meshed with the planet wheel. The planet wheel is meshed with an inner gear of the gear ring. The planet wheel is arranged on the planetary gear frame. The power output shaft is coaxially connected with the planetary gear frame. The retarder is connected with the gear ring. The differential motion wheel system is further provided with a locking device for locking or loosening the differential motion wheel system. By the adoption of the auxiliary transmission device, a vehicle applying a mechanical transmission does not need to conduct half linkage in the starting and low-speed running processes, and thus wear to a clutch is lowered; meanwhile, the auxiliary brake function of the retarder can be retained, brake efficiency is improved, wear to a brake system is reduced, and running safety is improved.