The invention relates to a method for enabling a solar water heater to discharge water at a constant temperature. The method is characterized in that a cold and hot double water inlet pipe thermostatic water heater arranged in the opening of a water outlet pipe of the solar water heater comprises a casing 1, an air vent 2, a water level sensor 3, an upper water inlet valve 4, a lower water inlet valve 5, a water outlet valve 6, a control panel 7, a main controller 8, an electric heating device 9, a water level adjusting valve 10 and the like, wherein an electromagnetic valve, a temperature sensor and a water flow sensor are respectively mounted on the upper water inlet valve and the lower water inlet valve; an electromagnetic valve is mounted on the water level adjusting valve; a temperature sensor and a water flow sensor are mounted on the water outlet valve; the water temperature required by a user can be set, adjusted and memorized by the control panel, and the temperature of the discharged water can be displayed. Through the adoption of the method, the solar water heater has the advantages that the solar water heater can be used immediately when being switched on, the water temperature is stable, and the solar water heater can be used in winter and summer, solar sources are better utilized, and the solar water heater is ecological and environmental-friendly.