Aiming at detecting the concentrated leakage passage in a dam, an entropy weight-variable fuzzy set model for dam leakage detection was developed based on information entropy theory and variable fuzzy set theory. Using a dam in southern China as an example, the indices to be detected were determined first, followed by the weight of each index via the entropy weight method in the objective weight assignment method. Then, the comprehensive membership degrees of different borehole water samples to precipitation, leakage water, and groundwater, as well as the H value of each water sample, were obtained. Based on these values, the borehole water samples were classified and their origins were determined. The results show that there are four concentrated leakage passages in the dam, which accords with previous findings.