In order to make steering operation of tracked vehicle simple, and decrease power reserve of engine, a set of mechanism controlled with steering wheel was designed based on the existing hydrostatic double flow differential transmission device. The rotational speeds of quantitative motors were controlled by angle of steering wheel and then distributed to outer and inner output shafts by mechanical transmission box, which solved the problem of auto-stepless deceleration during steering. Corresponding relation between rotational speeds of output shafts and angle of steering wheel measured by bench test was compared with the theoretical result. The results show that the auto-stepless deceleration is feasible by operating steering wheel of tracked vehicles during steering, the device can meet designed requirement, and provide a basis for theoretical analysis in studying steering transmission with double flow driving tracked vehicles.