Abstract: Since Oct 28th, 2009, the geoelectric field observation at Xuzhuangzi seismic station is interfered by square wave which degraded the observation data quality severely.In order to find the interference source, this paper firstly calculates the geoelectric field vectorial resultant to determine the azimuth of the interference source, then builds point source computation model according to Watson-watt Direction Finding Algorithm, calculates the distance between interference source and common electrode O, investigates electric equipments used in factories and schools in the direction of interference source.Through these methods, the equipment which leads to the interference is found out finally.It is confirmed that the electricity leakage of equipment produces severe interferences on the pattern of Geoelectric Field data, such as square wave.The interference factors are excluded thoroughly when the electricity leakage is controlled.