A squall line event on 28 April 2006 in Shandong Province was diagnos ed. The developing mechanism of the squall line was studied with the moist poten tial vorticity conservation theory. The results show that the squall line was ca used by 500hPa trough. The lowlevel temperature and humidity increasing, the c old air behind upper trough moving toward east and south and the low energy tongu e located above high energy tongue caused atmospheric convective instability. The air convergence and rising motion produced by shear line at 850hPa and the l ow pressure trough on the ground triggered convective instable energy release so a s to produce convective cloud cell which moved east and developed under the environment conditions of atmospheric thermal instability and strong vertical wind shear, thus forming a squall line. The lowlevel atmospheric moist baroclinity increasing destroye d geostrophic wind balance and resulted in slant vorticity developing, rising mo tion increased and convection came into being. Highlevel high potential vortic ity transporting downwards caused the lowlevel potential vorticity to increase , and to bring about vertical vorticity to increase. All the above factors are p ropitious to lowlevel mesoscale vorticity to develop so that convection increa sed. The interaction between strong rising movement and strong vertical wind she ar accelerated convection to develop and form squall line, and cause thunderstor ms and gales.