Abstract: Based on NCEP1 700hPa daily wind data, we draw a low-frequency synoptic weather map. First, we analyzed the low-frequency system on the low-frequency synoptic weather map, focusing on the low-frequency characteristics during the strong precipitation in the fl ood season over Shanghai. With the help of EOF method we made a thorough study of the spatial distribution of a low-frequency system. We took into consideration the integrity of a low-frequency system and the corresponding physical factors such as weather. The East Asia region can be divided into eight key areas. Then, by the concentration of a low-frequency system, 8 key areas are divided into fi ve active key areas and three less active key zones. For the construction of a low-frequency allocation model, we made a detailed analysis of strong precipitation in the fl ood season over Shanghai during low-flow field EOF from the decomposition of principal component space distribution characteristics. The results show that:North and south convergence of a low-frequency fl ow (warm and cold air) is the major factor that causes heavy precipitation in Shanghai area. In addition, the low-frequency cyclone located in China's eastern coastal areas and the low-frequency anticyclone located in low latitude regions of the Western Pacific are low-frequency systems that led to the strong precipitation process.