The invention discloses a horizontal hydrostatic extrusion reverse-extrusion device which includes an extrusion shaft, an end cover, a mold support, an internal mold, a pressure transfer medium, an outer mold, a first seal mechanism, a second seal mechanism, four tension columns and two baffle plates. Firstly, a blank to be processed is placed in the inner mold and is in interference fit with theinner mold. The inner mold is filled with the pressure transfer medium, the extrusion shaft extrudes from the end cover to the inner mold under driving of a hydraulic pump, and the pressure field having the pressure of more than 500 MPa is generated and extrudes the blank after passing through the pressure transfer medium; the blank is extruded out from a cover-free end of the inner mold, and a pipe is formed through a fixed diameter zone. Compared with ordinary extrusion, the formed product has better surface quality and better material performance; a polygonal pipe with complex cross sectioncan be extruded; the device is simple and easy to operate.