Two sets of hydrostatic transmission device matched with a set of comprehensive mechanical transmission system. were adopted to drive vehicle. When the vehicle is being driven straightly, only one set of device work. When steering is needed, the other begin to work, and their combined action in differential speed gearbox lead to the steering of vehicle. Unloaded bench test on interaction between properties of continuously variable transmission and steering wheel's angle was done, The result shows that theory of the auto-stepless reducing velocity is feasible by operating steering wheel of tracked vehicles during steering, and provides a basis for theoretical analysis on studying steering transmission with twin driving.