To study the variation law of the leakage and seepage drop due to the deterioration of water sealing in the vertical joints, a concrete face rockfill dam(CRFD)was taken as an example to simulate the seepage in the vertical joints on the left bank of the face slab, the vertical joints in the middle of the face slab, and the middle sag of the face slab under different elevations in the process of water sealing deterioration based on finite element method. The leakage through the face slab, the leakage measurement value of the measuring weir and the distribution of the seepage surface were calculated and the relationship between seepage gradient and the measured value of the measuring weir behind the dam was analyzed. The calculation results show that the measured value of the weir can reflect the deterioration degree of the water sealing in the vertical joints of face slabs and it can also estimate whether the seepage slope in the cushion zone exceeds the allowable value. With the deterioration of the water sealing, the leakage caused by the vertical joint in the middle of the panel increases, and the rise of the seepage line is more evident than that of the left bank. In addition, with the decrease of elevation, the leakage caused by the vertical joint in the middle of the panel increases, and the rise of the seepage line is also obvious.