A preliminary diagnosis on a severe convective storm (with hea vy rainfall, hail and strong wind) attacking Beijing urban zone on 1 August 2006 , by using the radar data retrieval method of lowlevel wind, temperature and r elative humidity based on the Variational Doppler Radar Analysis System (VDRAS) is s hown in this study with the results as follows. (1) The convective storm initiat ed in Hebei Province adjacent to the northwest mountainous area of Beijing, then it was organized and strengthened quickly while it moved into Beijing area under the condition of local thermodynamical forcing, and finally developed into a squall line. (2) A strong easterly flow persist ed at low levels in Beijing urban zone and southeast of great Beijing before the squall line initiated. The flow extended northwestward quickly and collided with across mountain northwesterly winds to trigger a boundary layer convergence line. The latter was the primary triggering factor of the squall line initiation. The inten sification, weakening and northsouth movement of the easterly flow at low levels had a distinct effect on initiation, enhancement and dissipation of the squall lin e, as well as persistence of the convergence line. (3) New storm cells initiated continually under the effect of the convergence line. (4) The intense convective storm went with a strong gust front.