Abstract: The paper aims to analyze the characteristics of water vapor source, vapor transport by jet stream and water vapor saturation of a winter rainstorm occurred from January13 to15 of 2012 in Guangxi, the authors used weather analysis and dynamic diagnostic methods to analyze the NCEP/NCAR 1°×1° reanalysis data, infrared imagery and other information. The results showed that: Bay of Bengal and the South China Sea were the two vapor sources of this rainstorm process, the establishment of low-level southwest jet stream at 500-700 hPa made the Bay of Bengal be the main source, strengthened the water vapor supply to the rainstorm area. The jet stream mainly depended on strong divergence effect of south side of west jet stream entrance of strengthening sub-tropical frontal area at 200 hPa. Different sources of water vapor transport had different heights, water vapor flux center appeared at 700 hPa. The vapor flux convergence layer was below 600 hPa, convergence at 700- 850 hPa was much larger than that at 925- 1000 hPa; the overlapping vapor flux convergence center at 700 hPa and 850 hPa was the rainstorm area, the increased height of convergence and the wet layer’severe scaling up could be the signal of the rainstorm. The conclusion contained the formation mechanism of water vapor conditions of a winter rainstorm, obtained prediction focus, and had a great effect on improving the accuracy of winter rainstorm’s forecast.