Abstract: Based on the NCEP 1°×1° reanalysis, intensive network of automatic weather stations and Doppler radar data, a squall line storm event occurred in Yunnan on 19 April 2016 was analyzed. In particular, the mesoscale characteristics of the formation of this storm were analyzed. The main results were as follows. This event occurred under the condition of a weak southern branch wave and shear line, coupled action of a high and a low level jet, and the downward transport of momentum. Low level vapor supply, mid-level dry air, inversion layer and the surface convergence line were favorable factors to the development of the storm. The mesoscale analysis of the radar echo showed that this event was influenced by the squall line, and lasted for a long time. The weak echo region behind squall line was quite clear. The radial velocity was characterized by the velocity ambiguity, the large velocity area and convergence in the low level, divergence in the upper level, and the mid-altitude radial convergence (MARC). The surface wind was closely related to the squall line, surface mesoscale convergence line and divergence. According to the surface convergence line, squall line could be used as a warning about an hour early. The above characteristics are of significant importance in thunderstorm gale detection and its early warning.