The characteristics of cloud cluster which brought the heavy rainfall in Beijing on June 24th 2002 is studied by use of satellites and radar data. The results show that during the weakening of meso-α convective systems, they may go through the evolution of meso-β scale convective cloud cluster systems which directly led to the heavy rainfall in the west of Beijing. Numerical simulation is done on the base of observational data analysis. Low-level eastern wind converged and rose on the eastern slope of Taihang Mountain where the low-level eastern wind moving westward from the Loess Plateau also rose slantwise by effects of topographic forcing, so strong vertical ascending motion generated on the Taihang Mountain and its eastern slope a secondary vertical circulation appeared. At the same time, low-level southern wind was ascending along the south slope of Yan Mountain where the cold air brought by northern wind was subsiding on the hillside, the forthcoming of this secondary vertical circulation was the main physical mechanism leading and maintaining the heavy precipitation on the west mountainous region of Beijing.