The monitoring of river discharge provides a data basis for the research of climate change， water resources monitoring， soil and water conservation， land water cycle， flood prevention and other aspects. It is of great significance for understanding the water resources and the surrounding ecological environment of rivers. Hydrological gauge stations exhibit a decreasing trend and uneven spatial distribution world-widely， and it is difficult to obtain continuous and complete flow data for many rivers. Therefore， here we present a method to extract river water extent using the Inland Water Monitoring System （SIMS） of Sentinel 1 data， obtain hydraulic parameters through integrating with digital elevation data， and then estimate river discharge using Manning's equation. River sections at Hequ and Fugu on the mainstream of the Yellow River were selected as case studies. Observed discharge at the corresponding gauges was used as the reference for accuracy assessment. The results show that river discharge was successfully retrieved at Hequ， with a Nash efficiency coefficient （NSE） up to 0.89 and a Root Mean Square Error （RMSE） of 37.92 m³/s. However， at Fugu， the accuracy is relatively lower， with a NSE of 0.10 and a RMSE of 135.60 m³/s. The main reason is that the river channel at Fugu has dramatic erosion and deposition， making the cross-sectional morphology unstable. Since all the input data of this method are globally accessible data， this study is expected to provide a new idea for monitoring discharge for ungauged river， and serve related hydrological and water resources studies.