The invention relates to a capping device of a conduit mouth of a ship body, which comprises a cylinder body 1, cover plates 2 and a cover plate driving device 3, wherein the 2-6 cover plates 2 which can realize 90-degree rotation are symmetrically distributed on the vertical circular section close to a cylinder mouth of the cylinder body 1 as far as possible, in the cylinder body 1, along the central line of the circular section, the rotation axial lines of the cover plates 2 are parallel to the central line of the circular section, and all the cover plates can just seal the vertical circular section of the cylinder body when the cover plates are closed; a driving rotating shaft 4 and a driven rotating shaft 5 are connected at the two ends of each cover plate 2, the driving rotating shafts 4 and the driven rotating shafts 5 are respectively fixed on the cylinder body 1 through bearing seats 6, the axial lines are superimposed, and the driving rotating shafts and the driven rotating shafts can rotate along the axial lines respectively; and the cover plate driving device 3 is fixed on the cylinder body 1 and can drive the driving rotating shafts to drive the cover plates 2 to realize the 90-degree rotation. The capping device can reduce acoustic noise generated by the open conduit mouth and improve the working conditions of a ship-borne acoustic detection system; and the structure is simple and reliable, the conventional relay control can be adopted, the operation and the maintenance are convenient, and the cost is low.