The invention discloses a novel mounting tool of an underwater horizontal connector. The novel mounting tool comprises an inverted U-shaped positioning plate, a driving plate and an aligning plate which are sequentially fixed on two parallel guide rods and can be sleeved on the to-be-mounted horizontal connector, a radial locking mechanism is arranged on the mounting tool and controls the horizontal connector to move along the radial direction, and the driving plate can move along the guide rods. When the driving plate moves along the guide rods, the driving plate can drive a driving ring on the horizontal connector to move. A power mechanism is arranged on the aligning plate and can drive the horizontal connector to move along the axial direction by the aid of power. According to a unique connector separation type hoisting scheme design, a connector body is put down and positioned roughly prior to that the mounting tool is put down and positioned precisely, and accordingly step-by-step implementation is beneficial to reduction of loss caused by misoperation.