The invention provides a hydraulic water retaining dam. The hydraulic water retaining dam has the advantages that a hydraulic cylinder swings flexibly, the service life is long and the fault rate is low. According to the hydraulic water retaining dam, the lower part of a flashboard is hinged to a dam foundation through a first hinged shaft; one end of the hydraulic cylinder is hinged to the surface, backing onto water, of the flashboard through a second hinged shaft, and the other end of the hydraulic cylinder is arranged in a concave part formed in the dam foundation and is hinged to the dam foundation through a third hinged shaft; the first hinged shaft, the second hinged shaft and the third hinged shaft are all arranged parallel to one another in the horizontal direction; an opening of the concave part is provided with a fixing plate for shielding the opening of the concave part; a through hole matched with the shape of the surface of the hydraulic cylinder is formed in the fixing plate; the height of the through hole is the same as the height of a shaft core of the third hinged shaft; and when the hydraulic cylinder swings in the concave part, the surface of the hydraulic cylinder is closely attached to the edge of the through hole; and a cylinder body swings in the concave part around the third hinged shaft to drive the flashboard to turn over, and the cylinder body is matched with the fixing plate all the time in a sealed mode so that impurities such as silt are prevented from entering the concave part, the rotation flexibility of the cylinder body can be ensured, and therefore the turnover of the flashboard can be ensured.