Abstract: The Wang4 well in Baodi of Tianjin City was began to be observed in April 1,1983, its observational values were relatively stable.The observational values of water radon in the well showed a sudden jump of a cluster of high values of more than 27.0 Bq/1, the highest one reaching 57.0 Bq/1, more than two times the normal values, the water radon values became discrete from stable ones,it has been more than two years up to now.After the high value appeared, the ppersonnel of the Analysis and Prediction Center and the Observation Center of the Seismological Bureau of Tianjin City did in—situ examinations for more than 20 times, made a lot of investigations and experimental work, and eliminated the relative interferences, we think that the high value variation of water radon is mainly the result of the influence of long term dewatering near Wang4 well.