Abstract: A kind of the hydro-mechanical split path transmission device for auto-stepless deceleration of the tracked vehicles during steering was introduced and its advantages were analyzed. The control theory of the auto-stepless reducing velocity and the theoretical data gained from the simulation method were introduced. It is believed that the new controlling device presented in this paper can increase steering radius under the same steering yawing rate and reduce the total dissipative power. Above all, it can complete spin turn at any vehicle velocity, and its controlling method is the same with wheel vehicles. Taking good advantage of the whole machine weight when it turns, there is no power cutting off, the traveling system can still output the total power of the engine to improve the operation flexibility.