A water heater comprises an inner container (1) and an outer container (2). An upper part of the inner container (1) is connected with an insulating water inlet pipe (3) which communicates with a cavity of the inner container (1); the outer container (2) is provided to encase the inner container (1) and it is axially movable relative to the inner container (1); two different heating electrodes (4) are connected with one end of the inner container (1) and one end of the outer container (2) respectively; water outlet holes (5) are provided on the inner container (1) and the outer container (2). As a result, the insulating water inlet pipe forms a hot water supply circuit in a sandwiched layer between the inner and the outer containers. By adjusting the spaces between the inner and the outer containers to regulate electrical current, water temperature control is achieved.