Using the data of rainfall in May from 11 counties in the northwestern Fujian from 1961 to 2014, conventional meteorological observation data, and NCEP 25×25° reanalysis data and so on, the atmospheric circulation characteristics and causes of extralarge May rainfall are analyzed. The results show that in the years of regionally and locally extralarge May rainfall, there were atmospheric circulation anomalies: 500 hPa highlevel Eurasia circulation from west to east shows a wave train distribution of “+-+”; the positive height anomaly zone to the north Okhotsk was stable, which played the role of blocking the eastward cold air; cold air along the northerly flow in the western side of the Ural before the ridge of positive height anomaly center moved southward continually; the negative height anomaly center in the Sayan area deepened, splitting eastwardmoving small troughs frequently at the bottom, which affected the northwestern Fujian; at 925 hPa, there was a stable shear line in the northern South China.The analysis results show the main cause include: at the 500 hPa East Asian trough shifted further east obviously, guiding cold air southward and meeting the southwest warm air on the northwest side of the strong subtropical high over northwestern Fujian; at the 925 hPa, there was a cyclonic convergence zone over the northwestern Fujian, and both north and south branch troughs were very active, moving eastward to affect the northwest Fujian.