The utility model discloses an expander for a cross-linked polyolefin vertical foaming furnace. The expander is constituted by an arc-shaped roller and a straight roller, wherein the arc-shaped roller plays a main expanding role and comprises a central metal rod piece, a pulley and a rubber elastomer housing, and the rubber elastomer housing can rotate around the axle center; and when the size of a master slice is increased in the foaming width direction at high temperature, the arc-shaped roller moves forwards, the master slice passes between the arc-shaped roller and the straight roller, the middle part of the master slice is jacked up by the arc-shaped roller, the master slice is expanded by the stretching action of the surface rubber elastomer housing to the two ends, and the folding and the adhesion of the master slice can be avoided. The expander disclosed by the utility model is simple to operate and convenient to use, and can effectively expand the foamed master slice; and furthermore, the surface of the master slice has no injuries basically, and the production efficiency and the quality of a product are improved.