The continuous rainstorm appeared in West of Southern Xinjiang from 14 to 19 June 2013. The convention observation, T639, NCEP reanalysis data are used in the analysis in order to know why the rainfall could be so much. Based on the relation between environmental air humidity conditions and the rainstorm, this paper analyzed the weather scale dynamic process and its forming reasons why the water vapor was transmitted over a long distance to Xinjiang, getting accumulated over the West of Southern Xinjiang. The results showed that rainstorm develops in an abnormal environment. During heavy rainfalls the top value of specific humidity reaches to 16-18 g·kg-1 in the lower troposphere. The extreme humid situation maintains even though the heavy rain appears, which is favorable for the longlasting severe rainstorm. The abundant water vapor is transported from Arabian Sea and Bay of Bengal to West of Southern Xinjiang by a southern flow. The southern wind increased the water vapor transfer. At the same time, with the southern airflow’s approaching Xinjiang, strong water vapor flux convergence is generated by lowlevel troposphere eastern jet stream in West of Southern Xinjiang, making the strong couple between upperlevel divergence and lowlevel convergence of vapor lower, and also strengthening the ascending velocity and inducing more wet air to higher altitude and then thickening the moisture layer further. The diurnal variation of the heavy rain is related to the vertical discontinuity of mesoscale circulation circle in Aksu southern and the lowlevel easterly jet enhancements at night.