Abstract: An infrequent excessive storm rainfall hits Beijing on 21 July 2012. The observed maximum rainfall reaches to 541 mm and the top precipitation intensity is 100.3 mm/h. The severe rain storm lasts for more than 10 h, causing a serious loss. The observation and the NECP reanalysis data are used in the analysis in order to know why the rainfall could be so much. Firstly, the relation between the moisture condition and the rainfall is reviewed. And then, the reason that the abundant vapor could be transported to and accumulates in Beijing is studied. The results show that this rainfall develops in a very wet environment. The top specific humidity reaches to 19 g/kg. And the value of this variable in the lower and mid troposphere is 40% higher than that of regional severe rainfall cases in Beijing. The extreme humid situation maintains even though the severe rainfall appears. It is favorable for the long-lasting severe rain storm. The moisture air is transported from the South China Sea and the Bay of Bengal to the Huabei areas by a southern flow which strengthens continuously to form a low level jet (LLJ). The core of LLJ moves northeastwards to Beijing gradually and leads then to an abnormal moisture flux convergence (-17.7 g/(hPa·m2·s)). At the same time, the strong couple between upper-level divergence and low-level convergence not only increases the convergence of vapor but also strengthens the ascending velocity inducing more wet air to higher altitude and then thickening the moisture layer further. These two synoptic dynamic processes result in the vapor being transported far away and accumulating in Beijing. There are two reasons which lead to the acceleration in the south flow. One is the current from a typhoon joining into the monsoon. The other one is the geopotential height gradient increasing in the mid latitudes, which is caused by a eddy in mainland going eastwards while the subtropical high in the Pacific moving little. Finally, a schematic chart about the main idea of the dynamic process is given.