An electric water heater having a filtering device includes an electric water heating device and a filtering device. The filtering device communicates with the electric water heating device, and includes a shell having at least an accommodating area, an inlet connecting to one end of the shell and communicating with the accommodating area, an outlet connecting to the other end of the shell and communicating with the electric water heating device and the accommodating area, a porous magnetic resin film located inside the accommodating area, and a calcium sulfite filter membrane in the accommodating area and between the outlet and the porous magnetic resin film. With the combination of the porous magnetic resin film and the calcium sulfite filter membrane respectively for the absorption of metallic impurities in water and chlorine removal, the scales can be avoided from generating after the electric water heating device heats the water.