Thunderstorms were generated in a weak vertical wind shear and strong unstable environment beginning from noon 1 August 2013 in Shanghai. During the developing process of the thunderstorms, three mesocyclones were detected, and especially during the formation of the second mesocyclone, characteristics of a classic supercell were found after the merging of two storms, such as lowlevel hook, pendant echoes, mesocyclone and TBSS (threebody scatter spike). Using conventional weather observations, dual Doppler radar data, automatic weather station data and wind profile radar data, the main formation mechanisms of the first two mesocyclone’s were found to be as follows: (1) the join of the previous thunderstorms’ outflow strengthens the environmental convergence and intensifies the lift of warm and moist air. (2) The outflow of thunderstorms strengthens the lowlevel vertical wind shear which is favorable for the formation of mesocyclone. In addition, due to the merging of the two thunderstorms, the intensified updraft and the strengthened rotation of the strom play important roles in the formation of the supercell mesocyclone. Then, during the formation of the third mesocyclone, the outflow of the storm was lifted by the downdraft of the adjacent storm, thus the rotation of the outflow and inflow in the mid and lowlevel was detected as the mesocyclone.