Anomalous characteristics of dynamic of oil wells in Shengli oil field before the 1975 Haicheng M_S77.3 earthquake,the 1976 Tangshan M_S7.8 earthquake and the 1989 Datong M_S6.1 earthquake etc.are analysed.Relationships between anomalous changes of dynamic of oil wells and origin time,place and magnitude for a coming earthquake are studied.Anomalous amplitude of ratio between oil and gas is larger than that of oil output of oil wells.The larger the anomalous amplitude,the larger the magnitude of coming earthquake.The shorter the epicentral distance,the larger the anomalous amplitude.The anomalies before earthquakes distant from the oil wells is a shape of sudden jump.The anomalies before earthquakes near by the wells arise more early with a complicated shape,which can be divided into early term trend change,mid-term vibrant change and sudden jump before earthquake in the main.Finally,mechanism of the anomalies is discussed.