Abstract: By analysing the characteristics of the four times of sudden bound anomalies and the relationship between the anomalies and the five earthquakes in Japan Sea,it is considered that the anomalies are those of "field precursor" before the five earthquakes in Japan Sea.The four times of anomalies are caused by the structural movement of the plate edge,which caused the change of the regional structural stress field in the area where the well lies.This leads to gushing of the gas in the oil bearing stratum under the water bearing rocks.