The invention provides an underwater exhaust port structure preventing water backflow. A power device positioned under a water surface is prevented from water backflow in underwater exhaust of a ship. The underwater exhaust port structure is characterized by comprising a horizontal straight pipe portion and a horizontal gradual change pipe portion, one end of the straight pipe portion is communicated with an exhaust gas discharging pipe of the power device, the gradual change pipe portion is provided with an initial end and a tail end, the initial end of the gradual change pipe portion is communicated with the straight pipe portion, the tail end of the gradual change pipe portion is an exhaust port, the vertical height of the gradual change pipe portion is gradually decreased from the initial end to the tail end, and the horizontal width of the gradual change pipe portion is gradually increased from the initial end to the tail end, so that the cross section of the tail end of the gradual change pipe portion is flat.