Using multiple intensified observation data and NCEP daily reanalysis data, the development mechanism of a nonlinear mesoscale convective system (MCS) that caused a short time severe rainfall in Xianning City on 14 July 2010 is analyzed. The results show that the organization of nonlinear MCS is poor during 13:00-18:00 BT 14 July 2010. The strong convective cells are disorderly distributed in big stratiform echoes, keeping the quasi stationary front staying over Xianning about 5 h, and thus leading to the short time severe precipitation in Xianning. The nonlinear MCS generates in a surface vortex circulation near Meiyu front, and the convections are triggered primarily by upper cold air and lifting motion in front of the front. The favorite ambient conditions for the nonlinear MCS to maintain and develop are the convergence and concentration of unstable warm moist airflow on the southern side of the shear line over the middle reaches of Yangtze River, the mesosacle convergence of surface local airflow and the favorable wind shear of boundary layer. In general, the high spatial temporal resolution observation data can be well applied in analyzing the MCS evolution process.