The invention is an extrusion concave mould of straight-tooth cylindrical gear, and its character: inlet diameter of the concave mould is greater than the diameter of divided circle of a molded gear and less than that of addendum circle of the molded gear; the shape and size of the gap between teeth at the end of the molding segment of the concave mould are consistent with those of the tooth form of planar projection of the molded gear; for arbitrary cross section in molding segment of the concave mould but point of tangency from the top arc of the tooth form to the side wall between teeth, the geometrical shapes and sizes of the rest parts between teeth are consistent with those of the corresponding parts at the end of the molding segment of the concave mould, and the centre angles opposite to the circumferential pitches are the same. It needs small extruding force, and the parts of tooth form are easy to fill up, the involutes of the concave mould is uneasy to wear and its service life is long.