Abstract: The South Asian high(SAH) in summer has an important effect on the distributions of drought and flood in the Middle and Lower Reaches of Yangtze River. In order to understand the interdecadal evolution of summer precipitation over the Middle and Lower Reaches of Yangtze River and improve the short-term climate prediction level, interdecadal variation of the SAH in summer and its impact on precipitation in this region was analyzed by using the NCEP/NCAR reanalysis data and precipitation data from weather stations during 1960-2015. The results indicate that there were remarkably interdecadal abrupts for the summer SAH intensity, area, south border, east point, west point and ridge line. Meanwhile a remarkably interdecadal variation of the summer precipitation in the Middle and Lower Reaches of Yangtze River also occurred. The SAH ridge line had the best correlation with the summer precipitation in the Middle and Lower Reaches of Yangtze River, showing a significant negative correlation. During the periods of summer SAH ridge line interdecadal southern, the SAH was relatively large and strong, and the upper air flow in the Middle and Lower Reaches of Yangtze River was strong and divergent. While the West Pacific Subtropical High increased westward extension, and there was a stable low pressure covered India. The Middle and Lower Reaches of Yangtze River located in the northwest side of the subtropical high and in front of the westerly trough where the prevailing southwest wind met the westerly airflow. The wind field in the lower layer had obvious shear and convergence, and the water vapor transported from the Bay of Bengal and the South China Sea converged and rose in the Middle and Lower Reaches of Yangtze River, resulting in heavy precipitation. From the early 1990s to the early 2000s, the summer SAH weakened and retreated in the west, while the subtropical high weakened in the east. However, due to the small degree of weakening of the two high pressures, the summer precipitation in the Middle and Lower Reaches of Yangtze River was still relatively high.