he precipitation weather systems were analysed by use of the historical weather chart of July and August in each year of the last 28 years,which cause the medium and heavy rain at Ihju Meng in Inner Mongolica autonomy region. The definition and properties of the low vortex of Qaidam which is the prime affecting system were discussed. The influence of mountain chain orientation on the occurrence and development of the low vortex,and the moving track and forecast of the low vortex were investigated.