Based on the observational data,radar echo,auto meteorological station data and NCEP reanalysis data of 1°×1°with 6h interval,a squall line weather event on March 22,2005 is conducted.The weather background,radar echoes,physics fields are diagnosed.The results show that the squall line occurred in the instable region。The outbreak of low level jet(LLJ)at 700hPa level and the core of the LLJ moving eastward quickly are the trigger of the squall line which is formed by the interaction of dry intrusion from mid-troposphere with the strong instability of low level.