Using multiple intensified observation data, detailed mesoscale and microscale analyses are undertaken on the severe convective systems in the early hours of September 7, 2008 over Beijing. Results show that, before the thunderstorms develop, Beijing lay in the right rear quadrant of 200 hPa jet streams and also in the southwesterly on the south side of the 850 hPa warm shear. Combined with the accumulation of unstable energy due to the low level warmmoist air advection, appropriate low level lifting mechanism or triggering would raise severe convective systems therein. Those MCSs, which moved eastward into Beijing, developed into severe convective systems again, after encountering the surface convergence line caused by urban heat island effect and the boundary layer disturbance caused by uplift and back roll of the topographic convergence, leading to a torrential rain event in the urban area of Beijing. Moreover, in northern Beijing, new thunderstorms were triggered by the surface convergence line, which was formed by the colliding between the outflow in front of surface high and the mountain breeze.