The routine observational data, the NCEP 1°×1° 6 h analyzed data and Doppler radar information were used in comparing two heavy rain processes under the posttrough on 7 May and the priortrough at 500 hPa on 14 May 2010 in Guangzhou.The results show that the 7 May torrential rain was caused by the shear line at 850 hPa and the northwesterly air flow after the trough at 500 hPa, which increased stratification unstability when it moved over the warm and wet air mass at low levels, this differs from the traditional circulation during the presummer flood season in South China. The “14 May” torreitial rain happenned under the influences of the southwesterly air flow before the trough at 500 hPa, the shear line at 850 hPa and the coldair injection on the ground surface. The squall line affected these two processes, in which the former had longer squll line, the larger area of the strong echo in squll line, more super cells and stronger rain intensity and higher total precipitation. The mechanisms of maintaining and development of the squall line in two processes were different in that, the former relied on the supplement and merging the new cell from the border of Guangdong and Guangxi, and the latter depended on absorbing and merging the new cell ahead of it or along the convergence zone on the ground surface.