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.