In August 2020, Minjiang River, Tuojiang River and Fujiang River saw great or catastrophic floods brought about by heavy rainfalls in the upper reaches of the Yangtze River. Although the reservoir group in the upper reaches of the Yangtze River has been fully involved in the flood control, peak reduction and peak shifting, floods above safety water level still hit the Zhutuo-Cuntan segment of the mainstream in the upper reach of the Yangtze River, posing huge flood control pressure to Sichuan-Chongqing reach. We quantified the benefits of operating the Jinsha River reservoir group to the flood control of Sichuan-Chongqing reach by investigating in-depth the floods in August 2020 from aspects of the overview of storm flood, the operation process of Jinsha River reservoir group, and the restoration calculation. Our findings demonstrated that the Jinsha River reservoir group had detained and impounded 4.737 billion m³ of water during the Yangtze River No.4 and No.5 floods, accounting for 57.7% of the total water detention by reservoir groups upstream of Cuntan, together with the successful peak shifting along the Jialing River and Minjiang River basins. It had also prevented the whole upper mainstream from exceeding safety water level by reducing the peak discharge and water level in Sichuan-Chongqing reach by 9 400-11 700 m³/s and 2.6-2.8 m respectively, and furthermore effectively relieved the flood control pressure by cutting the flood peak water level at Cuntan by about 2.6 m, equivalent to nearly 90% of the total reduction of peak water level.