With regard to failure paths in concrete gravity dams, which have similar characteristics to seepage channels, the cross-section damage method for evaluating concrete gravity dams is proposed on the basis of a two-dimensional percolation transition renormalization group. First, according to the percolation transition model of square units with equal local damage probabilities, the unequal local damage probability model is determined. Then, the trapezoid and triangle primitive cell damage types are determined. The percolation transition model with triangular sections is established. Finally, taking a concrete gravity dam section as an example, the probability of cross-section damage is analyzed. The structure is divided into three parts: the upper, middle, and lower parts. The local damage probabilities of the three parts are unequal. The results of iterative calculation show that when the local damage probability is less than 0.40, the dam section is safe. When it varies from 0.40 to 0.60, the overall safety decreases, with a transition from safe to damage overall. When the probability is greater than 0.60, the overall safety is very low. Cross-section damage may occur if the external environment worsens.