A water tank includes a box, an air valve body, a valve core, a cap, and an elastic element. The box defines an accommodating space and a vent communicating with the accommodating space. The air valve body is extended out from the box and adjacent to the vent. The valve core is mounted to the valve body to airproof the vent, and comprises a first end inserted into the accommodating space through the vent and a second end opposite to the first end. The elastic element is arranged between the cap and the valve core. The valve core will open the vent to deform the elastic member and allow heated air in the accommodating space to leak out of the accommodating space through the vent, in response to an air pressure in the box is greater a reference air pressure.