Using multiple surface observational data at high spatial and temporal resolutions, in conjunction with high resolution data produced using the Doppler weather radar three-dimensional digital mosaic system and the four-dimensional variational assimilation retrieval technique, the severe squall line event that occurred over the Yellow River and Huaihe River basins during 3－4 June 2009 is analyzed, focusing on its formation background and mature-stage features. The strong northwest flow behind the upper trough and the local warming of surface layer due to clear sky solar radiation provided large scale conditions for the formation and accumulation of convective instability energy. The dry ambient atmosphere at the low level with a large temperature lapse rate was favorable for the occurrence of disastrous gale at the surface during the squall line event. The evolution of the squall line and the associated severe convective weather phenomena during its life cycle is described. The surface thunderstorm high and the gust front developed into the most intense ones at the mature stage, when the area the severe convective weather phenomena occurred was largest. At its mature stage, the horizontal size of the squall line system reached the upper limit of meso-β scale, with double bands of strong radar echo inside. The features of cloud and weather phenomena associated with the two bands differed significantly. Based on the retrieved wind field, the vertical distributions of horizontal divergence and vertical velocity at four subregions of the squall line system, namely, the weak echo region before the squall line (P), the strong convective echo band in the squall line main body (C), the secondary strong echo region in the stratiform region (S), and the transition zone (T) are analyzed. There is descending motion within subregions T and S at the middle- and low-levels, with the descending in subregion T being stronger. The stronger downdrafts and thinner layer of divergence at low attitudes within subregions T and S are closely related to the disastrous gale at the surface.