4.78。 A new differential steering mechanism used in tracked vehicle that adopted hydro-mechanical double power has been studied. Calculating method and experimental method of steering power ratio for tracked vehicle of differential steering were presented and the model has been tested. The experiments showed that the new mechanism could realize tracked vehicle turning at random radius, and that no brake power loss when turning at small radius through two tracks rotate in reverse directions. The steering power ratio changed smoothly with inputting rotate speed and turning radius, the ratio is from 1.63 to 2.64 when radius is from 2.38m to 0.6m within big radius, the ratio is from 3.09 to 4.78 when radius is from 0.36m to 0.25m within small radius. A conclusion can be drawn that the maximum steering power ratio is approach 4.78 when tracked vehicle adopting the steering mechanism because the radius is equal to 0.25m that approach the minimum radius in pivot steering.