In this paper,the full moment tensor solution of M_S5.1 earthquake in Ningjiang,Jilin on May 18,2019 is extracted from the regional waveforms recorded by the Digital Seismic Networks of Jilin,Heilongjiang,Liaoning,and Inner Mongolia Autonomous Region with ISOLA program package. The focal mechanism solutions(nodal plane Ⅰ:strike=304°/dip=81°/rake=26°,nodal plane Ⅱ:strike=210°/dip=65°/rake=170°) of the Ningjiang earthquake show that the earthquake occurred as a result of strike-slip faulting at the shallow centroid depth of 6km with M_W5.0. The Husdon diagram shows the earthquake has an obvious non-DC component with volume expanding. Combining with the regional geological tectonic setting and a number of previous achievements in the source areas from the receiver function,electromagnetic sounding,and underground thermal structure,we infer that the geophysical process of the thermal fluids caused by the subduction of the western Pacific plate might change the Moho formation,which acts on the active faults to generate the M_S5.1 earthquake in the Ningjiang area.