A strong earthquake with magnitude of 6(3)/(4) occurred in Longxi county on March 1,A.D.138,which was the first one recorded by the Zhang Heng seismoscope.The event is made a textual research analysing historical data.The result shows that epicentre of the event with intensity of 9 degrees is between Minhe county of Qinghai Province and Lintao county of Gansu Province,and its geographical location is 35.9° north latitude and 103.3° east longitude.Regional tectonic background of the event is studied also.