The invention discloses and provides a horizontal straightening machine which is reasonable in design, high in straightening efficiency and high in adaptability to size ranges of workpieces, and can straighten two flanges of an I-shaped steel or H-shaped steel flange plate at the same time. The horizontal straightening machine comprises a control table, a feeding raceway group and a straightening main machine arranged in the middle of the feeding raceway group, wherein a flange straightening mechanism and a hydraulic transmission mechanism are arranged on the straightening main machine; the flange straightening mechanism is arranged above the feeding raceway group; and the feeding raceway group and the hydraulic transmission mechanism are connected with the control table. The horizontal straightening machine is applicable to the field of straightening and processing of steel structure workpieces.