The southwest vortex (21-23 July, 2008) moved northeastward from east of Sich uan. Henan Meteorological Observatory published rainstorm forecast at 21st with reference to several numerical weather prediction (NWP) commonly used, and the f orecast showed regional deviation. For the further understanding on the mechanis m of the moving route of the southwest vortex, it is discussed in some aspects s uch as atmospheric circulation, thermal, dynamics based on the formal and NCEP d atasets. Synoptic test on numerical forecast products has been done, and the pro cess is simulated by WRF. The results show that: (1) the route of the southwest vortex changes sharply, firstly southward, then northeastward, and then eastward , finally northeastward. (2) The structure of the vortex is of mesoscale charact eristics. K index and axial direction of the vortex have instructions on the moving of the vortex. (3) The main reasons of the deviation are more rely on the NW P and less consideration to the influence of the cool air from northeast. (4) Th e forecast of the vortex’s route by WRF is better.