To overcome the limitations of existing methods for diagnosing the deformation state of high arch dams, a new diagnosis method was proposed considering the progressive failure characteristics of the deformation instability process. The method combines modern dam engineering, mechanics and mathematical theories. The elastic deformation state of high arch dams was calibrated based on the characterization of water pressure, temperature change, and time-dependent deformation. The individual diagnostic index was established using the information entropy theory. According to the projection pursuit method, the individual contributions were optimally achieved, and then the group diagnostic index was built. The diagnostic control values were constructed based on the typical small probability principle. On this basis, the diagnostic criteria of group normality, group basically normality, individual abnormality, group abnormality, and group deterioration were developed. Engineering example results indicate that the established method can effectively characterize the progressive failure process, and it is applicable to the situation that the most unfavorable load has not been encountered yet.