Abstract: Cumulative sum (CUSUM) control chart is a more mature method for outlier identification, which can effectively identify and forewarn the deformation information in the global navigation satellite system (GNSS) coordinate series, but for long-term continuous monitoring, gross errors inevitably exist in the GNSS coordinate series, which has a certain impact on the accuracy of the CUSUM control chart. Therefore, a modified CUSUM control chart based on median was proposed to identify and forewarn the deformation information in the GNSS coordinate series, and its principle and algorithm flow were given. The results show that the accuracy of deformation information identification and forewarning using modified CUSUM control chart is better than traditional method, and the false alarm rate of the disaster warning are reduced, which shows the necessity and superiority of using the modified CUSUM control chart.