Abstract:The intensive observations of automatic weather station, Tianjin MM5 m odel products, the NCEP 1°×1° of 6 hours analyzed data as well as conventiona l Doppler radar information are used in the weather diagnosis and mesoscale analys is for the process of a heavy rainstorm on the August 26, 2007 in the eastern re gion of Tianjin. The results show that heavy rains occurred in the large scale circulation to the zonal adjustment period. The affecting systems are the troug h on 500 hPa, the shear line on 700 hPa and 850 hPa and ground mesoscale convect ive line. The main reason of the uneven heavy rainfall distribution is obvious s cale in various physical characteristics. Ramps were formed over air flow of the storm zone which led to stable development of a strong thunderstorm cloud, at the same time the easterly storm on ground transmitted a lot of water vapor. The existence and maintenance of the ground mesoscale convergence line not only enhance he convergence uplift, but also cause the current to the large scale s torm in the system and trigger conditions. The obvious train effects in Doppler radar map, as well as correspondence between extended height of heavy echo and i ntraday zero height are the main reasons for the heavy rainfall.