The invention discloses an assembling tool and method for a clamping ring in a blanking cap of a pipe connector. The tool comprises a cone, an outer cylinder and a positioning hole. The positioning hole is formed in the bottom of the cone. According to the assembling method, the blanking cap is arranged in an outer sleeving nut, a bearing pad is placed at the lower portion, the positioning hole of the clamping ring assembling tool is inserted in the outer circle of the blanking cap, then the clamping ring is placed in the cone of the clamping ring assembling tool, the clamping ring downwards slide to be assembled in a ring groove of the blanking cap, and assembling is finished. The assembling tool is simple in structure and convenient to use, the assembling method is rapid and flexible, practicability is high, clamping ring assembling becomes very easy, time is greatly saved, and the work efficiency is improved; and in addition, after the clamping ring is arranged, scratches cannot be generated on the outer circle surface of the blanking cap.