The hydraulic oil freely flows among each group of oil cylinders to distribute the oil quanity, forming the gradient heights, so as to enable the self-propelling trailer to keep the leveleness of loading surface on the uneven road. If without control, all oil cylinders of the self-propelling hydraulic flat trailer will be parallel connected together, which will cause the vehicle body unbalanced and cannot work. So, the hydraulic system of vehicle is divided into three groups of relatively independent hydraulic subsystem (forming three-point support), and each group of subsystem will be driven by the power pump in the power station.