Abstract: The background of climatic anomalies relating to heavy rainfalls over the middle reaches of the Yangtze River in China are studies in this paper. A t-test for composite signal field is performed. The case studies of heavy rainfall show that the distribution of significant anomaly in the 500 hPa signal field is an important precursory signal of heavy rainfall. The pattern of the signal anomaly with ‘+-+' in the composite signal field in the cases is found. It occurs in the area over eastern part of China.