Abstract: Miyun, the northeast county of Beijing, had a local and short-time heavy rain on 1 August, 2002. The amount of precipitation reached 280.2 mm and the rain caused a mud-rock flow and flood fatality. Satellite cloud pictures show that this rain was triggered by a mseo-βconvective system which settled in the northern mountain area. The dense isoline region of TBB and the up-rushing cloud top indicate the rainfall area. The large-scale circulation shows that there was sinking motion, accompanying an inversion layer which restrained the convection and favored the accumulation of instable energy. During the rain process, the conversion from sinking to ascending made the convection possible. The large-scale analysis of water vapor indicates that there was abundant water vapor from the northwest of the subtropical high to Beijing. The result of surface mesoscale analysis shows that the mesoscale low and the convergence line are trigger factors for a mseo-βconvective system, and that the high-humidity and high-temperature airflow is in favorable of local heavy rains.