A squall line weather process occurred in Shanghai on July 12 2004 is diagnosed with various kinds of observational data, such as the automatic meteorological station data, satellite water vapor image and Doppler radar echo, and NCEP reanalysis data of 1°×1°with 6h interval. The results show that Shanghai is located in an unstable stratification and the dry and cold intrusion down from the upper troposphere and the strong convergence of the moist and warm air in the boundary layer are the trigger mechanisms. The radar echo shows that the squall line is shaped by the joint type of line of discontinuity and line of rearward continuity. The bow echo belt causing thunderstorm and gust in Shanghai is the connection of the convective cells and the new ones appearing on the right of them.