Abstract: The wild values deviating from the target truth values often occur in the safety monitoring of dams.To effectively detect the quality of observation data and to diagnose the wild values,the causes for the wild values are analyzed,based on two kinds of techniques,i.e.,the residual analysis based on statistics and the stochastic fuzzy diagnostic technique based on probability.To verify the rationality of the method,the present method was applied to the analysis of monitoring data during the construction period of Yanshan Reservoir,and satisfactory results were obtained.