The invention discloses an assembly tool for an O-shaped sealing ring. The assembly tool is characterized by comprising an external diameter enlarging section, a connecting rod section and a guiding section which are integrally formed, wherein the external diameter of the external diameter enlarging section is gradually increased from the tail end to the direction of the connecting rod section; the external diameter of the tail end of the external diameter enlarging section is smaller than the internal diameter of the O-shaped sealing ring; the connecting rod section is positioned at the front end of the external diameter enlarging section; the external diameter of the connecting rod is greater than the internal diameter of the O-shaped sealing ring and is greater than or equal to the external diameter of a connector to be mounted; the guiding section is positioned at the front end of the connecting rod section; and the external diameter of the guiding section is smaller than that of the connecting rod section, so that the guiding section is inserted into the connector to be mounted. The O-shaped sealing ring is conveniently assembled to the connector to be mounted by pushing the sealing ring from the tail end with smaller external diameter of the assembly tool until the sealing ring is expanded after reaching the part with greater external diameter, and thus the labor intensity is reduced and the assembly efficiency is increased; and the sealing ring is not easily scratched, the service life is long and the sealing effect after assembly is ensured.