The invention relates to a flat-pushing type side-tip dumper. A head is arranged in the front of a frame, a wheel mechanism is arranged at the lower end of the frame, a carriage and a flat-pushing power device are arranged on the frame, the carriage comprises a bottom frame, stand columns, movable side doors, side plates, a movable rack and end plates, the bottom frame is a rectangular frame structure consisting of long sides and wide sides, a cross brace is arranged in the bottom frame, chutes are formed in the cross brace and the top surfaces of the wide sides of the bottom frame, the movable rack which moves relative to the bottom frame is arranged on the bottom frame, a bottom plate of the movable rack consists of a plurality of bottom secondary plates which are arranged in parallel inthe width direction of the bottom frame, each two adjacent bottom secondary plates are hinged to each other through hinges, the hinges are positioned on the bottom surfaces of the bottom secondary plates, sliders are arranged on the bottom surfaces of the bottom secondary plates, the sliders are matched with the cross brace and the chutes formed in the wide sides of the bottom frame, and the flat-pushing power device pushes the movable rack to move along the chutes. The flat-pushing power device pushes the movable rack away from the bottom frame from the side plates of the bottom frame, the bottom plate is opened under the effect of the weight of cargoes, and thus, flat-pushing self-discharging of the dumper is realized.