Abstract: Based on the 1951－2005 monthly rainfall data in Northwest China, which is provide by the Chinese Meteorological Administration Data Center, the authors analyze the spatial and temporal patterns of JJA rainfall in Northwest China. The author try to explain these patterns by using water vapor transport and atmospheric circulation by ERA40 reanalysis data. Results show that the rainfall increase in the west part of Northwest China is highly related to 500-hPa geopotential height changes in Siberia (50°N－60°N, 40°E－70°E) and the Iranian plateau (35°N－42.5°N, 60°E－75°E). Water vapor convergence is another important factor especially after 1987. The rainfall decrease in the west part of Northwest China can be modified by the sea surface temperature. Atmospheric circulation systems, such as the North Atlantic Oscillation (NAO), the North Pacific Oscillation, the Niño 3.4 SST, and the vortex area index, are discussed with typical anomalous patterns of summer rainfall. It shows that the NAO and the vortex area have some relations with year-to-year changes of the first EOF mode of rainfall in Northwest China. As the correlation between the first principle component and the vortex area index is high, the vortex area can be used as an indicator to predict the rainfall change in Northwest China.