A water tank of a heat storage type solar water heater is provided with a tank body formed by an inner container, a casing and an insulation layer. A water inlet pipe and a water outlet pipe are arranged in the tank body, a heat storage clamping cover covers the inner container, heat storage paraffin with the fusion point of 55 DEG C- 65 DEG C is arranged in the heat storage clamping cover, the insulation layer is arranged between the heat storage clamping cover and the casing, and a heat storage paraffin inlet and a heat storage paraffin outlet are formed in the heat storage clamping cover. The heat storage clamping cover filled with the heat storage paraffin covers the inner container, therefore, when the water temperature in the inner container is higher than the fusion point of 55 DEG C- 65 DEG C of the paraffin, the paraffin absorbs heat to fuse; the water temperature in the inner container drops in cloudy days or at night; when the water temperature in the inner container is lower than the fusion point of 55 DEG C- 65 DEG C of the paraffin, the paraffin begins to solidify and emits heat; therefore, the water temperature in the high-temperature chamber rises, and water temperature drop time is prolonged.