Abstract: The No.4 well at Wenjin Street in Hengshui City showed a large amplitude anomaly of water temperature in September 1991.By water quality analysis and the general investigation of the well, it-is preliminarily proved that the large amplitude increase of the water temperature had no relation with earthquake.Then, by investigation on the range of scatter of thermal anomaly of water temperature, in-situ experimental observation and analyzing the downhole water-extracting equipment, the real reason for the water temperature increase has been found out.The thorough investigation of the anomaly has stabilized the production and living orders, and produced a good social result.