The utility model discloses an amplitude expander for a vertical foaming furnace. The amplitude expander is characterized by comprising a flattening roller and a positioning roller, the flattening roller is a non-equant straight roller, and the middle part is provided with a convex camber surface; the flattening roller is capable of moving along horizontal direction and vertical direction in the furnace body and capable of rotating around self axle; the positioning roller is an equant straight roller or a non-equant straight roller and below the flattening roller, and capable of rotating around self axis under the driving of a motor, and capable of horizontally moving and moving up and down; two ends of the flattening roller and the positioning roller are respectively fixedly connected with outer walls at left side and right side of the vertical foaming furnace body through a flattening roller base and a positioning roller base. Compared with the universal amplitude expander for the existing foaming furnace, the amplitude expander disclosed by the utility model is simple in structure and convenient for operation, and the surface of a foaming master slice cannot be damaged, the product production efficiency and quality are improved.