The invention discloses an assembling tool of a jacking hydraulic oil cylinder piston. The assembling tool of the jacking hydraulic cylinder piston comprises a ring body, the ring body is formed by splitting a plurality of ring modules, and a splitting surface is formed between every two neighboring ring modules; the plane of one of the splitting surface is located passes by the center of the ring body, and the planes of other splitting surfaces are inclined to the plane of the splitting surface passing by the center of the ring body; the outer diameter of the ring body is identical to the inner diameter of the portion of a cylinder barrel where a groove is formed, the inner diameter of the ring body is identical to that of the portion of the cylinder barrel without the groove, and the width of the ring body is 0.2-0.5 mm smaller than that of the groove, so that the ring body can fill and level up the groove of the cylinder barrel. The assembling method of the jacking hydraulic oil cylinder piston comprises pushing the piston a Y-type seal ring across the position of an original groove; prying the gap between the first ring module and the second ring module to jack the third ring module out through the first ring module and the second ring module; after the third ring module drops out, taking the first ring module and the second ring module out. The assembling tool of the jacking hydraulic oil cylinder piston can achieve rapid replacement of the Y-type seal ring and avoid damage to the Y-type seal ring.