Abstract: Firstly, sea surface temperature eigenvector fields in North Pacific for each month over Jan-May and cross spectra of summer rnonsoon in the middle and lower reaches of the Yangtze River for June were analysed from 1956-1975. The result shows that the eigenvectors will each go in phase with the total correlation coefficient of the summer monsoon, oscillai.ing in consistent cycles, if the former distribute similarly each other.Further correlation analysis indicates that the influence of anomalous thermal wind, caused by meridional gradients of the sea surface anomaly, changes the intensity of West Pacific zonal wind and the position of sub-tropical high, resulting in consequent change in the intensity of summer monsoon in the middle and lower reaches of the Yangtze River.