A case study of an EF2 tornado that occurred in Wenchang, Hainan on 5 June 2016 was carried out by using observation data, automatic weather station data, Haikou Doppler radar data, wind profile data and 2G high resolution visible cloud images of Fengyun Meteorological Satellite. The results showed that: (1) This tornado was triggered by a supercell, corresponding to the edge of subtropical high, front of trough at 500 hPa, shear line at 850 hPa and the south side of ground thermal low. The favorable environmental conditions for the developing of the supercell were the significant increase of vertical wind shear in 0-2 km and the lower lifting condensation level, both caused by sealand breeze, and the quick increase of CAPE with solar radiation heat. (2) Both the mesoβ scale convergence line (seabreeze front) and the outflow boundary of thunderstorm triggered and enhanced the developing of the supercell, which was on the east side of eastward squall line, spreading from east to west along the mesoβ scale convergence line, and in contrast to the average wind direction of the storm bearing layer, that is, backpropagating. Some characters of the supercell including hookshaped echo, highlevel echo overhang, mesocyclone and small scale severely shear (similar with TVS) were observed. The middle mesocyclone tornado funnel cloud was generated during extension strengthening, touchdown to low level. The tornado reached its strongest stage during the time of the small scale shear developing both upwardly and downwardly from the middle level. Tornado occurred at the maximum gradient area of weak echo reflectivity of low level. (3) The merge of sea breeze front line and sinking air outflow border of supercell, generated the breeze front located in the south of supercell, and finally a horizontal vorticity tube formed from east to west orientation. A upright vorticity tube twisted when the horizontal tube met the updraft near the weak hooklike echo, forming a lowlevel mesocyclone. The breeze front located in the south of supercell met with eastward moving gust front, and the boundary layer convergence was enhanced, favoring lowlevel mesocyclone to gain the stretch rotating acceleration, and eventually tornado formed.