Based on NCEP reanalyzed data and Doppler radar products, a squall lin e process in outer periphery of tropical cyclone “KAMMURI” on August 4, 2008 w as analyzed. It showed that the establishment of the lowlevel jet and temperat ure rising caused by sinking and downdraft within subtropical high created the s ufficient water vapor and unstable condition in South China. The squall line weat her burst out under the action of the boundary convergence line which was caused by weak cold air. The appearance of the weak echo channel behind the arc echo s howed that the squall line attacking the airport was in the most powerful stage. Thunderstorm, gust front and downburst were together with the sudden change of the wind direction and the sudden decrease of the ground visibility. Vertical se ction and wind shear radar products were used to analyze the lowlevel wind shear threatening the safety of flights.