Abstract: In the context of MM5, this paper investigates the numerical simulation of cloud mot ion wind data assimilation (CMWDA) with its impacting on a torrential rain over low-mid Yangtze reaches in 1998. Evdence suggests that CMWDA can availably modify the zonal wind intensity in front of high level jet, and it have an obvious improvement on quality of upper wind. In such a way as to get more accrate precipitation field. There is no doubt that the use of cloud motion wind data is an important approach to enhance the rainfall forecast in operation and research work.