A heavy rainfall occurred in Beijing on June 25,2005 is quite different from the weather forecast. With the data from satellite,radar,observations,wind profiler and automatic weather station, a study is made. The event is not a typical heavy rainfall, but it has clear mesoscale characteristics. The results show that low-level weak cold air from the north, low-level southwesterly jet stream and mesoscale convergence line in the south are the main factors.