Abstract: This paper uses Sentinel-1 data to obtain the monthly water area of Shanghai-Yibin section from 2016 to 2020, and analyzes its annual and inter annual changes. The results show that:①Monthly variation rules is that January to May water surface variations are relatively stable, and the water area in June to August has gradually increased and is peak in July. It's falling slightly in September, and gradually decreases to stability after reaching the peak in October. ②Seasonal variation rules is that the waters in winter is the smallest area, the summer water area is the largest, It shows a significant seasonal difference in summer and winter. ③Endevelopment rules is that the trend in the water area after 2016 where the water area is relatively stable in 2017-2019, and is growing in 2020. In terms of segmentation, the water area is downstream with time to change from time to time, the range of which is biger than the middle's, and the middle's is biger than the upstream's, the upstream changes are relatively stable, and the downstream is more remarkable. ④It is prone to flooding, which mainly is distributed in a mid-range reaches, and it is necessary to pay attention to monitoring warning.

Key words: Sentinel-1, Google Earth Engine, remote sensing dynamic monitoring, mainstream of the Yangtze River