Anomalous characteristics of observation data of 6 wells in Liaohe oil field before the Xiuyan M_S5.6 earthquake,Liaoning on Nov.29,1999 are analysed.Observed items are groundwater level,oil recovery volume,casing pressure and gas recovery volume.Trend,short term and impending anomalies appeared in observation data of the 6 wells before the event with fairly good synchronism and a shape of rise-fall-rise.The test shows disturbance factors,such as water flooding,pumping water,atmospheric pressure and temperature etc.have not influence on the observation data,therefore,the anomalies can be believed.