Perennially suffering from weathering erosion and manmade damage, openair immovable cultural relics face severe problems.Present condition based trend prediction is part of the "preventive" protection concept. However, sometimes, because of uncertainty in the prediction method, little useful information is obtained.Traditional statistical probability methods and fuzzy comprehensive evaluations are not suitable for making prediction models. We used the Tang Dynasty Shunling Tianlu stone carvings in Shaanxi as examples for predicting fracture damage based on grey system theory. Two models, the GM(1,1) and the Verhulst models of settlement are presented. The grey coefficient and the development coefficient are calculated by ordinary least squares. Experimental results show that the average predicted relative errors for the two models are 6.23% and 4.40%, respectively,and meet the expectations of crack prediction accuracy. This research provides a quantitative basis for assessing the health of relics and for guiding future research.