On July 17, 2007, a major rainstorm occurred in the north slope of the Tianshan Mountains. The analysis results show that the circulation background of the rainstorm displays a twobody pattern of South Asia high and stabilization of low system in the middle of Asia. On the condition of steady stratification, the systematic ascending motion in the both sides of the Tianshan Mountains, the northern airflow in low levels, and the orographic uplift all augment rainstorm precipitation. The rainstorm area is corresponding to the cyclonic vortex column, with the vapor transported to the rainstorm area from Bay of Bengal through high-level southwesterly winds and lower northeasterly winds. There is a close relation between positive potential vorticity anomaly and the rainstorm.