The west and north parts of Beijing area are mountainous and the urban heat island effect is obvious in Beijing area, these may influence the distribution and intensity of the rainfall. Using the intensified observation and analysis data including wind profile data, temperature elements of surface automatic weather stations (AWS), the wind distribution and vertical divergence of Variational Doppler Radar Analysis System (VDRAS ), the effects of topography and urban heat island on a mesoβ torrential rain in Beijing area are studied in the afternoon on July 9, 2010. Result shows: (1) When the wind is toward the mountain, the rainfall gets stronger at the windward slope; (2) The urban heat effect is obvious before the rainfall begins, the convergence at low levels because of the effect is about 1000 m high, and at the same time the temperature gradient caused by the urban heat effect can cause vertical wind shear, so that both the convergence of low level and the vertical wind shear can lead to the grow and development of the convective system; (3) Once strong convective precipitation begins on the windward slope, the positive feedback between rainfall intensity and southeast wind velocity appears, and the process is an essential condition to form mesoβ scale torrential rain.