According to statistics, there are 17 heavy rain cases happened in the south of Zhejiang Province in May—June of 1960—2002. Using NCEP reanalysis data, the synoptic situations of 17 cases are analyzed, and are divided into some synoptic patterns. Its physical characters, including Q-vectors, are also calculated. The relationship between rainstorms and Q-vectors is discussed in detail. The results show that the patterns of heavy rain primary based on temperature are concise and practical. The convergence of Q-vectors and rainstorms are well homologous,the center of rainstorm caused by non-thermal depression situated in the convergence center of Q-vectors or south to southwestern of the convergence center of Q-vectors. It has good effect for rainstorm forecast in May—June of 2005.