The invention discloses a positioning assembly device which is used for mounting of an on-shaft seal ring (8) of a countershaft (9). The positioning assembly device comprises a sleeve (5), a taper ring (6), a spring (4), a retainer ring (7) and a positioning ferrule (3). The top end of the sleeve (5) is provided with a hole with the diameter smaller than that of a cavity of the sleeve (5); the outer wall of the taper ring (6) is closely attached to the inner wall of the sleeve (5), taper ring (6) is capable of moving axially along the sleeve (5), and the center of the taper ring (6) is provided with a taper hole with the diameter smaller than that of the bottom end; the spring (4) is arranged in the sleeve (5) and positioned between the taper ring (6) and the top end of the sleeve (5); the retainer ring (7) is arranged in the sleeve (5) and positioned in a cylindrical opening at the bottom end of the taper ring (6); and the positioning ferrule (3) is sleeved on the sleeve (5), and the inner wall of the positioning ferrule (3) is attached to the outer side wall of the on-shaft seal ring (8). Using the positioning assembly device for mounting the on-shaft seal ring, scratch damage between the on-shaft seal ring and a shell can be prevented, and weakening of tightness caused by scratch of the on-shaft seal ring by the shell due to improper mounting can be avoided.