The invention discloses a mechanical valve with automatic cut-off function which can be applied in the production process of chemical materials, comprising a valve body, a valve seat, a valve core, elastic parts, a pneumatic mechanism and a magnet; the valve seat and the valve core are arranged In the valve body, the valve core is used to contact the valve seat to close the valve; the elastic member drives the valve core to maintain a distance from the valve seat; the pneumatic mechanism includes a cylinder, a piston, a piston rod and a pipe body, the cylinder is set in the valve body, and the piston is set in the In the cylinder, the two ends of the piston rod are respectively connected with the piston and the valve core, and one end of the pipe body is connected with the inner cavity of the cylinder, and the other extends to the outside of the valve body; the magnet is set on the side of the valve core away from the valve seat; the initial In the normal state, the valve core is adsorbed on the magnet. When the amount of gas entering the cylinder from the pipe body reaches a certain level, the air pressure of the gas will cause the valve core to separate from the magnet and then quickly fit on the valve seat. After using this valve, the flow cut-off of the valve occurs instantaneously, so there will be no situation that the flow rate becomes smaller and smaller during the flow cut-off process.