A squall line that occurred in eastern China on August 17, 2009, was successfully simulated with the Advanced Regional Prediction System (ARPS) by assimilating high-resolution Doppler Radar data and conventional observations. Using the simulation output, we analyzed both the dynamical and thermodynamical characteristics of the squall line and diagnosed the environmental conditions of a back-building convective line that was a part of the squall line. Our major conclusions are as follows: (1) during the formative stage of the squall line, the southward-moving cold air from the low-level mesoscale convective vortex (MCV) in a clustered convective system, which was located to the north of the squall line, encountered southeastern warm moist air. This triggered strong convections and initiated the squall line. As the squall line propagated south, the role of the cyclonic circulation associated with the MCV in the maintenance of the squall line convection was reduced. Instead, cold surface outflow associated with the cold pool, which is an inherent part of the squall line, played a critical role by converging with the ambient air flow from the south. (2) The evolution of the squall line was enhanced by four convective lines that were formed by back-building convective cells. Several dynamical and thermal processes resulted in environmental conditions that favored the formation of the back-building convective cells. These conditions included: a small quantity of convective inhibition, a large amount of convective available potential energy, moderate vertical wind shear, strong low-level convergence, and large helicity.