Abstract: The focal mechanism solution of the largest earthquakewith magnitude ofM_L3.4 of the Hebei Huailai earthquake swarm sequence is calculated using P wave polarity data and the fault plane of the focal mechanism solution is determined combining with distribution of aftershocks:strike 151°, dip angle 83°, slip angle 11°. The NW-trending Huangtuyao-tumu fault is the seismogenic structure of the earthquake swarm which is caused by both the regional stress field and Huailai-zhuolu basin north margin faults. According to its earthquake swarm sequence parameters, the Huailai earthquake swarm sequence belongs to atypical precursory earthquake swarm. In recent years, the seismic activity of the Yanqing-huailai basin is in a low level and show fault-locking characteristic;therefore, attention should be paid to the seismic risk in the area. But the earthquake swarm gives little instructions for short-term earthquakebecause it is not a precursory earthquake swarm.