A particularly challenging event occurred on 22 July 2009 in Beijing area. The convective storm moving from the mountain to the plain suddenly intensified as it approached the foothills, and then intensified again as it moved onto the plains. Based on the fourdimensional Variational Doppler Radar Analysis System (VDRAS), this paper presented a deep investigation focusing on the intensifying mechanism of the convective storm under weak synopticscale forcing over Beijing area using Doppler radar, wind profile, surface observation, 4 times daily radiosonde and NCEP data. Results have shown that: The weak cold front triggered the convective storm. The thermodynamic unstable condition including warm, humid, lowlevel southerly wind and significant accumulation of the convective available potential energy (CAPE) indicated by the sounding over plain, and the significant dynamic instability with strong lowlevel vertical wind shear indicated by the wind profiler provided the favorable local environment for the intensification of the storm. In the first intensification stage, the terrain forcing played a dominant role, including: (1) acceleration of cold pool outflow whose kinetic energy drived from potential energy, leading to strong down flow and strong convergence; (2) uplift of south warm moist air flow at the south of piedmont, resulting in a strong upward motion; (3) lifting of the thunderstorm cold pool outflow, which formed strong lowlevel vertical wind shear between north outflow at the low level and south air flow on the surface and led to cold advection over the warm and moist southerly flow, thus enhanced both the dynamic and thermodynamic instability in front of the storm. The second intensification of the thunderstorm in the eastern urban area, was mainly caused by the presence of the strong perturbation temperature gradient zone in the boundary layer in Chaoyang District, resulting from the confrontation of the cold pool outflows of the wellorganized thunderstorm and the lowlevel warm tongue, and the balance between positive and negative vorticities. The intense updrafts pushed by the interactions between the cold outflow and the warm moist southerly flow, coexisted with the downdraft at the rear of the thunderstorm for a long time. The negative vorticity due to the vertical inhomogeneity of the outflow, was balanced with the positive vorticity due to the vertical inhomogeneity of the environmental flow. According to the RKW theory, this equilibrium state was very favorable for the formation of vertical updraft so that maintained the storm and led to the second intensification.