A ML3.8 earthquake occurred on February 23，2014，in Rongchang County，which is located at the southern edge of the Sichuan Basin in the border area between Sichuan and Chongqing． This paper presents results of the focal mechanism solution of this earthquake using the CAP (Cut and Paste) method based on broadband seismograms recorded by regional seismic stations in the Chongqing area． Our results showed that the moment magnitude and focal depth are MW3.09，and ～ 3 km，respectively． The hypocenter of this earthquake is located close to a buried fault in the Luoguangshan anticline． Oil prospecting and deep drilling data indicate that this berried fault is a thrust fault，striking SW230°，dipping NW45°，and ～ 1.7km deep． There are some injection wells within the anticline，and significant injection － induced earthquakes were observed during the periods of injection of waste water into the deep formations through those wells． One nodal plane of the best double couple solution of the ML3.8 earthquake shows strike 247°，dip 48°，and rate 104° (46°，44°，74° for another nodal plane)，in agreement with the geometry of the buried fault． Therefore，we conclude that the ML3.8 Rongchang earthquake occurred as a result of fault slip along the berried reverse fault reactivated water injection under the compressive stress regime in the area．