The invention provides a metal water tank electric water heater provided with a water flow sensor with a pressure reduction function. A water tank 8 is made of metal materials. The water flow sensor 4 with the pressure reduction function is installed on a water inlet pipeline of the water tank. The water flow sensor is composed of a component A and a component B. According to the component A, when water flows, an impeller or a rotor or a Hall element 4-8 is pushed by water to rotate, pulse signals or current signals or voltage signals and the like are output by sensing the water flow rate, an intermediate relay is pulled in, and a circuit is closed; and when the water supply is cut off, the circuit is opened. According to the component B, a transverse through hole is formed in the small end of a piston 4-9, and a through hole is further formed in the middle of the piston 4-9; when water inflows, water passes through the transverse through hole and then enters the water tank through the middle hole of the piston 4-9, and water is prevented from being blocked in the water inlet process; and when the water tank is filled with water or the water inlet amount is larger than the water outlet amount, the pressure in the water tank is increased, back pressure is generated by the big end of the piston 4-9, the piston is made to move, and accordingly the area of a water inlet 4-11 is decreased or even the water inlet 4-11 is closed.