The invention relates to the field of supporting systems, in particular to semitrailer supporting system improvement and provides a semitrailer supporting system ingenious in structure, high in connection stability, great in limiting performance and convenient to adjust. A semitrailer comprises a balance beam, a gooseneck, a rear suspension and a rear frame. The gooseneck is connected above the balance beam in a floatable manner; the rear suspension comprises a fixing frame and at least one axle, and one end, away from the tail of the semitrailer, of the fixing frame is fixedly connected with the rear frame; the rear frame and the fixing frame are connected above the at least one axle in a floatable manner; a front horizontal valve is arranged on the top surface of the gooseneck, and a rear horizontal valve is arranged on the top surface of the fixing frame. The semitrailer supporting system has advantages that problems of deviation and even breakage and the like of the at least one axle in the rear suspension are avoided to the greatest extent, and accordingly stability and safety of the semitrailer are greatly improved.