Precipitable Water (PW) data obtained from retrieving observed data, is used to improve the initial humidity field with the variational assimilation method. The numerical simulation experiments are carried out with MM5 using different initial fields of a heavy rain event happened on 9 to 10 July 2003. The results indicate that the data assimilation of PW has a strong capability of adjusting the initial humidity field; after PW assimilation,the quality of the initial humidity field can be greatly improved,and the accuracy of precipitation forecast can be effectively enhanced.