The Weather Research and Forecasting(WRF)model coupled with a single-layer urban canopy model(UCM)is used to simulate numerically the summer urbanization effects with high resolution during 2003-2007 over the Yangtze River Delta. The comparison of two experiments with urban land cover(URB) determined by MODIS satellite observations and without urban areas(NOURB) shows that the water vapor mixing ratio near the surface in urban and adjacent areas decreases obviously, while at higher level (850-700 hPa), it increases. By comparing URB and NOURB experiments of water vapor mixing ratio at 2 m in rainy days at each level, the difference in light rainy days is closest to that in all summer days. The results of the analysis on the circulation, divergence field and vertical speed show that the changes in vertical structure of water vapor mixing ratio is due to stronger lower-level convergence and higher-level(850-700 hPa) divergence caused by urbanization, which enhances convection in urbanized areas and makes the vertical transportation of water vapor more active, so as to bring more convective precipitation.