Abstract: To detect gross errors in dam safety monitoring data,mathematical morphology filter is used to detect gross errors.The filtering algorithm and the selection of structuring element are studied considering the features of monitoring data.First,we select an appropriate structuring element according to the features of dam monitoring data;then,we perform mathematical morphology filtering to monitoring data according to the selected structuring element;finally,we compute the residuals between the measured value and the filtering result,determine the threshold for gross error detection using quartiles and detect gross errors using the threshold.The filtering result shows that the adverse influence of gross errors is effective,and all the added gross errors are detected.