Abstract: Using the data from conventional weather observations, the Shanghai WSR-88D Doppler weather radar, and observations from automated meteorological stations, a long-lived severe squall line on July 12, 2004 is analyzed. The case analysis shows that the intensification of the jet between 850 hPa and 500 hPa that causes the low level frontogenesis and high potential severe weather. The squall line triggers in the case are the surface front, the low level convergence, and upper level divergence. The strong vertical wind shear and the feedback between the strong updrafts and downdrafts in the storm sustains the squall line in a severe stage for 6 hours. The interaction between the squall line and the outflow boundaries of the storms along the same Mesoscale Convective System (MCS) intensifies the other storm and changes its path。