The present invention relates to an instantaneous heater, comprising a heating assembly, wherein the heating assembly includes a heating element, a first shell, a first inlet tube, a first water outlet, a hot water cavity, a cold water cavity and a capillary tube, the hot water cavity is in the first shell, the heating element is in the hot water cavity, the hot water cavity communicates with the cold water cavity, the first inlet tube communicates with the cold water cavity, the first water outlet is on the top of the first shell and communicates with the hot water cavity, the capillary tube is in the cold water cavity, one end of the capillary tube communicates with the hot water cavity and the other end communicates with the outside of the first shell. The instantaneous heater boasts simple structure, low cost and effective prevention of vapor from rapidly erupting.