The invention discloses a die for the lower cover of a water heating tank. The die comprises a plurality of ejector pins, wherein the ejector pin located at the corner of the die has a diameter of 10 mm. According to the die for the lower cover of the water heating tank in the invention, the diameter of the ejector pin is changed from conventional 6 mm to 10 mm, so the ejector pin located at a place where an ejecting force is great has sufficient strength, and the ejector pin of the die hardly deforms and does not lead to bad quality of a product; production efficiency is improved, and production cost is reduced.