A type of mixed energy heater with constant temperature control, which belongs to the field of water heater technology. This water heater includes a gas heating portion, an electric heating portion, and a control circuit. The control circuit includes a power output controller. The electric heating portion is composed of water passage and electric heating device installed in the water passage. The water passages of the gas heating portion and the electric heating portion are connected together in series or in parallel, and the terminal of the passage is equipped with a temperature sensor. The output of the temperature sensor is connected to the signal input of the control circuit.