The present invention relates to an assembly for blocking a cooling water channel in a mold, which includes a bolt with a threaded-shank and a head, a base with a threaded-hole for thread-fastening the thread-shank, and an elastic ring, an elastic ring-fixing member, a pressing ring, an expansion ring, and a seat member which are disposed between the bolt and the base from the base to the bolt. The elastic ring-fixing member has an elastic ring seat holding the inner side of the elastic ring, the inner side of the elastic ring is fitted in the elastic ring seat, a body is made by insert-molding nylon to protect the base with the threaded-hole and composed of an upper cylindrical portion, a lower cylindrical portion larger in outer diameter than the upper cylindrical portion, and a step formed at the joint of the upper and lower cylindrical portions.