Using Doppler weather radar, satellite, other observational data and the WRF numerical simulation output, the squall line event that occurred on the boundary of Jiangxi, Hubei and Anhui Provinces on 19 June 2016 is analyzed, focusing on its evolution and the dynamical and thermodynamical characteristics. The results are as follows. (1) The squall was combined and enhanced by several convective cells, and the cloud cluster that causes the heavy rain is almost steady. (2) The development of the squall line is related to the surface converging lines and the cold pool which is formed by cold air mass sinking from higher levels and latent heat cooling during water evaporation. The surface convective lines triggered and organized the convective cells in the formative stage of the squall line. Cold surface outflow associated with the cold pool converged the ambient air flow from the southwest that made the squall line form with a back-propogating feature. (3) The low-level vertical shear, which is vertical to the squall line, enhanced the development of the severe convective cells that served as the conversion between the storm inflow and the horizontal vorticity. (4) The storm relative helicity is used to measure the strength of the inflow to the storm and the horizontal vorticity along the direction of the inflow. The storm relative helicity was increased by the development of the squall line and reached the maximum in the mature stage, which was conducive to the convective system, and then decreased with the dissipation of the squall line.