Based on the WRFVar assimilation system developed by the U.S.and the cloud drift wind information of China,the threedimensional and fourdimensional variational data assimilation is performed on a single case of heavy rain in the valley of Yangtze River on 10 June 2008.Results show that the two kinds of data assimilation of cloud drift wind can improve forecast accuracy in heavy rain and torrential rain.The fourdimensional experiment scheme works particularly well in improving the accuracy of torrential rain.This is considered the most important in precipitation forecast.The further analysis of initialization of two assimilation tests shows that the assimilation of cloud drift wind can well improve the initialization,and has positive impact on the prediction of the physical variables.Overall,the use of fourdimensional assimilative scheme can more effectively adjust the initial field,and reduce the forecasting errors of various elements.The causes of heavy rain are also discussed.Results show that this torrential rain process occurs under the combined effects of upper trough and lowlevel shear line as well as the highlow jet.The lowlevel jet provides not only water vapor needed by the rainstorm,but also energy for positional instability of stormforming convection.