Abstract: A dynamic diagnosis and meso-scale analysis of an infrequent squall line on March 22, 2005 in Guangdong is made using the routine observations, automatic weather station data, NCEP/NCAR reanalysis data and Doppler radar images. It's found that the squall line occurs under unstable stratification of environmental conditions and unstable physical mechanism, such as the trough at 500 hPa affecting the region where Guangdong borders Guangxi ahead of the trough at 700 hPa, so the acclivitous trough may be the dynamical trigger mechanism of the squall line's occurrence; the intrusion of the dry and cold air down from the upper troposphere affords the thermal instability field; the westerly jet is overlapped with the low southwest jet over the Guangdong and Guangxi, which brings the strong vertical wind shear, and the squall line develops strongly along the exit area of the low level jet after it is formed at the entrance of the upper jet above the low level jet; in the north and south of the squall line there are many new convection cells which keep building and tend to be combined towards the middle of the bow-shaped echo when the squall line grows; the squall line tends to have a dissy mmetrical structure rather than a symmetrical structure when it weakens, and the comma head and tail of the squall line, which cause disasters, are still growing respectively; the echo's channel of weak reflectivity factor at the rear end of the bow-shaped echo, namely the mesosphere-influx mouth of trough, comes forth, which is a sign to the change when the squall line has turned to dissymmetrical structure from symmetrical structure, and the squall line develops to the most powerful stage; in this process it shows some features such as the bow-shaped echo, the V-shaped mouth of trough, MARC and "long convection line" in front of the squall line. The strong inflow center, velocity convergence zone and small vortex velocity feature from the meridional velocity are also found. It is a good sign to identify the change of multi-cell storms that the long convection line of images occurs and develops all along the squall line; diagnostic analysis shows that the physical measures such as the vertical speed, wet static energy and CAPE can reflect effectively where the squall line has been born and where it has moved to.