The invention relates to an instant electric water heater, in particular to a heating device of the instant electric water heater. The heating device of the ultra-thin instant electric water heater comprises an outer tank body with a water pipe inlet and outlet channel, a PTC (positive temperature coefficient) ceramic is taken as a heating body and is positioned in the outer tank body, the PTC ceramic is in a sheet shape and is clamped by sheet type electrodes which are arranged on the top and bottom surfaces of the PTC ceramic, have consistent size and are connected with wires, and a polyimide film used as an insulating material is coated outside the electrode. The heating device is safe and reliable, is thin and can be washed after being disassembled.