Both the evil weather derived from the thunderstorm and the thunderstorm itself are severe threats to the flight safety. Based on the conventional data, satellite image and radar echo combined with diagnostic analysis of moist potential vorticity, a severe thunderstorm event occurred in urumqi inter-national airport on June 26, 2005 is analyzed. The results show that:(1) when the thunder-storm occurred, the surface features and the weather of the airport changed abruptly as follows:the temperature lapsed, the humidity rose surprisingly with the pressure surging,the strong wind and heavy shower followed subsequently. (2) The event was triggered by a squall line . The genesis of the squall line may have relations with the interaction between a cold meso-α scale high pressure and a short-life warm meso-β scale low pressure. (3) Accumulated unstable enengy released explosively after the cold air damaged the inversion layer, which blocked the upward transportation and diffusion of water vapour and heat. (4) The moist symmetric unstable region of 925hPa indicates the thunderstorm location in some sense, a strong thunderstorm may occur in the central region where MPV<0.