A new differential steering mechanism adopting hydro-mechanical double power used in tracked vehicle was designed. The new differential steering mechanism is made up of three planetary systems. Combining the power of the derailleur and that of the turning hydraulic system, the mechanism realizes two tracks turning in reverse directions. The experimental results show that it has good heading performance and the yawing rate is 07%, which accords with national standard. Quickly turning is achieved while only turning power input. With the increasing of inputting rotate speed, turning angle velocity is also increased. The neutral turning is realized and the minimum turning radius is 0.02 m as the tracks on both sides have different skids and slips. 