A local torrential rain process occurring in Tianjin on September 26, 2009 is diagnosed and a mesoscale analysis of it is performed, based on the conventional and surface densified automatic station observations, data of mesoscale model of TMMB, satellite cloud pictures and Doppler radar data etc. The results show that the heavy rain occurs when the largescale circulation pattern is adjusted from meridional to zonal. The trough of 500 hPa is the major synoptic scale system in this event, while the main mesoscale systems are surface mesoscale convergence line, humidity frontal zone and sea breeze front visible in Doppler radar imagery, which are driven by sealand breeze circulation. And a fully developed thermal instability before the occurrence of the torrential rainfall has being established, the synoptic scale environment has considerable energy storage, favorable dynamic condition and moisture convergence from the Bay of Bengal at 700 hPa and the sea to the east of Tianjin at 850 hPa. The collision of synoptic system driven cumulus convection and sea breeze front triggers the release of instable energy, resulting in the heavy precipitation of the first period. The strong easterlies in the boundary layer indicate the essential role of sealand breeze circulation in this torrential rain event.