Cracks in concrete dams are inevitable disease and whether they are stable or not is a key factor relating to the structural safety of the concrete dams. According to function relationship between crack opening and crack tip opening displacement, critical crack opening criterion of concrete dam crack abnormality diagnosis was proposed. The determining method of crack tip opening displacement and crack subcritical extension amount needed by the critical crack opening criterion was investigated. The critical crack opening criterion was transformed by using of crack opening monitoring model and associated with the crack in situ monitoring data in dam safety monitoring, crack opening to establish the order typical small probability diagnosis method of concrete dam crack behavior abnormality. The case study shows that the order typical small probability diagnosis method implement specific application of critical crack opening criterion in dam safety monitoring, and it is reasonable and feasible.