Abstract: A control method of a thermostatic system for a hot water heater is provided where when the water temperature in the hot water heater is too large, a first electromagnetic valve is controlled to be open and a second electromagnetic valve is controlled to be closed. Gas will be transferred via the first gas room to the second gas room. The control of the electromagnetic valves causes a decrease in gas outflow to the hot water heater. When the water temperature in the hot water heater is too small, the first electromagnetic valve is controlled to be closed and the second electromagnetic valve is controlled to be open. The further control of the electromagnet valves causes an increase in the open degree of a gas outlet valve, hence increasing gas outflow.