To improve the simulation accuracy of flow in ungauged small and medium rivers, river surface widths were inverted, and preliminary discharges were estimated using satellite imagery (Sentinel series and high-resolution images). These estimated values were then used as observed flow data and assimilated with the flow data simulated by the MIKE11 model using the Kalman filter algorithm. The assimilated flow results were fed back as updated inputs to the MIKE11 model, forming a closed-loop iteration that continuously corrected model errors and enhanced simulation accuracy. Validation results from the Huangkou Gate section in Quzhou County show that after data assimilation with the Kalman filter algorithm, the MIKE11 model achieves an R 2 of 0.820, an NSE of 0.813, and an RRMSE of 0.260 for simulated flow. Compared to pre-assimilation results, R 2 increases by 28.1%, NSE improves by 27.0%, and RRMSE decreases by 43.5%, demonstrating effective improvement in river discharge simulation accuracy.