The invention discloses a corner diameter expanding extrusion die and method for preparing a tubular product by adopting a bar and belongs to the technical field of metal plastic working. The corner diameter expanding extrusion die comprises an upper mold plate, an extrusion core rod, an upper base plate, a core rod fixing plate, an unloading plate, a concave mold, a concave mold supporting plate,a spring, a convex mold supporting plate, a lower base plate, a lower mold plate, an inner guide pillar and an extrusion convex mold. The mold adopts the floating convex mold, and the front end of the extrusion core rod is of a cone-shaped structure, so that a material flows favorably and the extrusion force is reduced; axial extrusion, secondary corner extrusion and diameter expanding extrusiondeformation of the tubular product can be achieved by means of an extrusion channel formed by the extrusion core rod and the concave mold. The mold can achieve extruding, unloading and ejecting operations of the tubular product sequentially in one stroke, so that the extrusion die is high in production efficiency and suitable for batch preparation of superfine crystal tubular products, and can achieve industrial applications of a severe plastic deformation technique.