Disclosed are a water storage tank bladder, a manufacturing method therefor, the water storage tank including the bladder, and a water treatment apparatus including the water storage tank. According to one embodiment of the present invention, the water storage tank bladder used in the water storage tank to receive and store water and discharge the stored water has a lower side of which the thickness can be thicker than the thickness of the upper side of the bladder and thicker than the thickness of the lateral sides of the bladder.