Abstract:A mesoscale analysis and diagnose of an extreme bursting torrential rain in northwest of Yuanling on May 27, 2008 is performed by using the routine observational data, TBB data and NCEP reanalysis data. The results show that the heavy rainfall occurrs in the favorable environmental conditions,low-layer southwest jet brings moisture and instable energy for the heavy rain area . The MβCS convective system is formed by two mesoscale convective clusters , it shows an ellipse-shape structure and maintains over the heavy rain area for a long time.The heavy rainfall clusters appear in front of MβCS convective system where the convection development is strong. Mesoscale systems which are corresponding with the activity of rain clusters can be clearly seen from the stream field after filtering,and the heavy rainfall appears in the lower layer convergence between two mesoscale convergence lines of 850hPa and 700hPa .The rainfall relates to the distribution and change of the center of low-level positive vertical helicity and high level negative vertical helicity over heavy rain area .The existence of a secondary circulation forced by the ageostrophic wet Q-vector is propitious to the release of instable energy and plays important roles in the process of the heavy rainfall.The specific topography of heavy rain area results in precipitation enhancement.