Abstract: The JL Concrete Faced-Rockfill Dam (CFRD) has a maximum height of 140.3 m and is constructed using granite and quartz porphyry rockfills. During its initial operation, the dam encountered issues such as water-stop damage, slab cracking, and excessive seepage, which affected its normal operation. This study analyzed the damage mechanism of the slab during the early operation based on settlement data and slab inspection records. By Using a 3D refined numerical simulation method, the deformation process of the rockfill dam and the stress evolution of the concrete slab were reproduced. Research indicated that the sudden drop of water level that occurred during the initial impoundment led to the damage of the slab water-stops, seepage erosion of the cushion led to gaps between the slab and the rockfill, which further deteriorated the stress of the slab, resulting in progressive damage and cracks. After lowering the water level, the damaged slab was repaired and the gaps were treated with grouting. The leakage of the dam has been controlled. The case of the JL CFRD gives profound enlightenment to the design and construction of CFRDs: The design and construction of the CFRD should ensure that the deformation between the material zones is coordinated so that the slab is in intimate contact with the cushion; During operation period, sudden drop of water level should be avoided as much as possible to prevent waterstops damage and loss of cushion material. The filter criterion of the cushion zone, transition zone, and rockfill zone should be strictly satisfied to avoid the loss of fine particles of cushion material once the slab cracked, which will lead to further damage of the slab.