To facilitate the maintenance of hidden frame glass curtain walls, it is necessary to identify the damage location of the structural sealant accurately. However, detecting concealed sealant damage directly is challenging. This paper proposes a two-stage damage localization method to pinpoint the center of damaged structural sealants. In the first stage, an assurance criterion of boundary modal curvature (ACBMC) is developed to identify damaged edges. In the second stage, the exact damage location is determined by comparing the differences in boundary modal curvature between the undamaged and damaged edges. Two-panel units are analyzed by numerical and model experiments. The results show that the proposed method can effectively identify the damage location of the structural sealant when the damage severity is higher than 4%.