Aiming at the problem that the existing dam disease detection methods can only roughly locate the area where the crack is located， a dam crack extraction method based on a universal target detector is proposed. Firstly， a two-target detector is designed to detect the crack area and the water stain area as two independent targets on the image at the same time. Secondly， the geometric position relationship between the crack area and the water stain area associated with the same crack is established. Finally， the upper boundary of the water stain frame contained in the crack frame is uniformly sampled， and the curve fitting is performed on the sampling points to obtain the crack curve. The experimental results show that the proposed algorithm can not only accurately detect the crack frame and water stain frame， but also fit the crack curve completely， and it has been effectively verified in the detection of dam diseases with millimeter-level width.