Abstract: Remiiu oil field is located in Renqia county of Hsbei Province and the Raoyiang seg, the subtectOnic element of central Hebei dpression in North China plain.Because of its special stress and geologic condition, the region has becoras a weak posttio.i in geological aad mechanical structure.With the exploitation and the water injection at high pressure in the oil field, the effective stress further decreases to induce the small swarms, of which the maximum magnitude is M_s=3.3.#br#Before moderately strong earthquakes in North China, the small swarni activity has show an obvious raising trend.Such feature coincides vith the occurrence of small swarms before great earthquakes with magnitude 5-7 in North China.The region is a sensitive point of reflecting cfustal stress change under the action of general stress field in the la,ge range of North China block.So it can be considered as a "window" for monitoring the change of seismicity in North China.