The invention discloses a water draining method for a narrow estuary U-shaped bay bridge construction. The method comprises the following steps: (1) a dam is filled in an estuary of an U-shaped bay; (2) firstly, the riprap construction is adopted to perform the doundation treatment; and then, soils and stones are used for filling, and are compacted to form the dam; (3) a gate is mounted on the dam; the bottom height of the gate is lower than the lowest water level of seawater on daily; the gate is closed at high tide in every day, and is opened at ebb tide; the inner water level of an internal lake is exhausted lower by using the tidal action to form a low-water level internal lake; when the water level is lowered to the ebb tide water level, the gate is closed; a construction working surface is built in the internal lake; and a bridge is built on the construction working surface; and (4) after the project construction of the internal lake is finished, the gate is opened at high tide, and the seawater outside the dam is guided in to increase the water surface height of the internal lake; and when the water surface height of the internal lake is close to the external water surface height, the gate, the dam and piles are disassembled. The method reduces the filling earth volume or the material utilization amount of steel pipe piles.