Abstract: Using upper and ground based observations data，by means of Cressman’s objective analysis and composite analysis method，the large scale features of persistent rainfall which occurred from 10 to 20 July，2011 over the whole Yangtze-Huaihe region （26-34°N，110-122°E，including the Yangtze River and the Huaihe River valleys） are studied，and the rainstorm occurred in this period from 11 to 13 is dynamically diagnosed. The results are as follows：（1） There are double ridge of high pressure and one trough around 50-70°N in the large scale circulation field，the western one is over the Ural Mountains （at 40°E），the eastern one is over Yakusk （at 130°E），and it is the trough over the Lake Balkhash between them，as well as nontypical double-blocking high type. As the trough is moving eastwardly and deepening，the short-waves trough which split from the bottom of it brings about the heavy rain. The Western Pacific Subtropical Anticyclone is by eastern and its ridge is by northern. A deep trough over the southeast of Lake Baikal extends to the Southern China from Mongolia，the southwestward airflow of the ahead of the trough impacts the Jianghuai Basin steadily. （2） The Yangtze-Huaihe region is located on the right side of the high level southwest jet’s entrance area and the nearside of the low level south jet. The dry cold air comes from the mid- high latitudes and the mid-high level，the dry cold air intrusion contributes to the formation and maintenance of the dry leve1. The dry cold air intrusion is one of the important dynamical and thermal causes for the formation and maintenance of moist front during the Meiyu period. （3） The water humidity comes largely from two sources，one is at the Bay of Bengal whose humidity is transferred by westerly flow on the south side of the monsoon depression via the Indo-China peninsula into the Sea and advancing northward into the Yangtze-Huaihe region，and the other is over the South-China Sea which offers water vapor for the rainband mainly through SW monsoon. （4） The cyclonic vorticity and convergence at the low level，the anticyclonic vorticity and divergence at the upper level，deep upward motion plays a main role in the maintenance of sever precipitation occurred in 11-13 July，2011. Based on the moist potential vorticity （MPV） theory，the vertical and horizontal components of MPV，MPV1 and MPV2 are analyzed; symmetric instability is related with convective instability，inertial instability，moist baroclinicity and vertical wind shear. It is favourable to develop symmetric instability when MPV1<0，MPV2<0，the symmetric instability intensified the torrential rain. The conditional symmetric instability （MPV<0） provided favorable instability mechanism for the occurrence of rainstorm.

Key words: Yangtze-Huaihe region, persistence rainfall, large scale, circulation character