Abstract: The calibration of Tai'an DSQ water pipe instrument in the year-mid and year-end of 2020, there were many repeated calibrations in NS direction, which were unqualified. Analysis of the change of voltage difference in single terminal during calibration process, it is judged that the mercury in the calibration device fell, resulting in N-terminal blockage. The calibration is qualified after maintenance. In the process of typical fault investigation, the method of analyzing the variation law of single-ended voltage difference has a good reference and guiding significance for the operation and maintenance of station water pipe instrument and the identification of calibration fault.