The utility model discloses a from letting out formula overflow box, it includes the airtight cavity of rectangle (1), and discharge orifice (2) have been seted up to the upper surface of the airtight cavity of rectangle, and the adaptation has cylindrical cavity sealing member (3) in the discharge orifice, and the top rigid coupling of cylindrical cavity sealing member has the external diameter to be greater than the round platform shape shutoff portion (4) of discharge orifice, and the bottom of cylindrical sealing member is equipped with cavity drainage portion (5), and spout hole (6) have been seted up to the side of cylindrical cavity sealing member. The beneficial effects of the utility model are that liquid medium in the overflow box increases when causing the liquid level to rise, and cylindrical cavity sealing member will receive the effect of buoyancy and float for the spout hole that lies in the overflow box under the normality inside exposes, and liquid medium guarantees by this spout hole outflow overflow box, round platform shape shutoff portion that the spout hole is not disturbed by external rainwater, realizes the one -way flowing back of spout hole promptly, if have gas in the overflow box and can discharge yet from this, can not introduce external rainwater when guaranteeing the overflow effect yet.