The 900X emergency shut-off valve belongs to a kind of valve that is intended for use in a water supply system in which the fire fighting water and domestic water are connected in parallel and for the control of the direction of water supply. When a fire breaks out and a large quantity of water is in great need for fire fighting purpose, the 900X emergency shut-off valve cuts off the domestic water supply immediately so as to ensure enough fire fighting water. When the fire fighting is over and the pressure in terms of water use has dropped, the valve switches on automatically to stay in the constantly open state and the normal domestic water supply is restored. This valve makes it unnecessary to set a separate fire fighting water supply network in the system, thereby greatly reducing the construction cost and water consumption. Further more, the valve is high in control sensitivity, safe, reliable, simple to debug and offers a long service life.