Using the data of conventional and automatic weather stations (AWS), Doppler radar, satellite and wind profiler radar （WPR） as well as the associated NCEP reanalysis data (1°×1°), this paper analyzed the convection conditions and mesoscale characteristics of an extreme rainstorm in Beijing and its surrounding areas on 21 July 2012. The results suggested that sufficient moisture， convective instability, continuous “train effect” of the convection systems and a low mass center precipitation echo structure with high precipitation efficiency were the main causes for the generation and development of the extreme heavy rainfalls. It also showed that the newborn and severely developing convective cells were located at the intersection part of low level shear line and surface convergence line. The movement direction of convective cells and the presence of “train effect” can approximately be judged according to the direction and speed of the mid level winds as well as the position and direction of surface convergence line. Mesoscale analysis on the severe precipitation was made based on satellite infrared cloud images and radar reflectivity data, and the results showed that there were three stages in the process of the rainstorm: (1) The first stage is the generation period of the convective systems before their strongly developing; (2) The second stage is the most severely developing period in which extreme rainstorm can be observed by most meteorological stations in Beijing; (3) The third stage is significantly diminishing period of the severe convection and precipitation.