The invention discloses a water blocking cap press-mounting assistive tool which comprises an assistive tool press head. The assistive tool press head is provided with a shaft body, a front end and a tail end. The front end is a stepped shaft. A groove is formed in the top face of the front-most end of the stepped shaft, is concave in the axial direction of the assistive tool press head, and is internally provided with a magnet. The tail end is provided with a knocking cap which is provided with a knocking end face and used for being knocked. A water blocking cap is attracted by the magnet in the groove and prevented from falling off in the mounting process, the front end is the stepped shaft to ensure that the end face of the water blocking cap is tightly attached to the assistive tool press head, and the consistency of the mounting depth can be ensured through contact of the stepped face of the stepped shaft and a to-be-mounted face.