Small hydropower has the characteristics of small capacity, multi-point distribution and strong basin correlation. The traditional follow-up power generation mode is disorderly, and the flexible and adjustable value of small hydropower groups has not been fully utilized. The evaluation and optimization of adjustable capacity is the key to support the peak regulation of power grid and the accommodation of wind, solar and electricity. Based on the runoff prediction of small hydropower groups in the basin, it is proposed that the output corresponding to the maximum power generation flow of the current water level of small hydropower is the upper limit of adjustable output, and the output corresponding to the ecological installed capacity is the lower limit. The adjustable capacity interval of small hydropower is determined, and then the adjustable capacity optimization model of small hydropower groups in the basin is constructed and an improved particle swarm optimization algorithm is proposed. Taking the five series runoff power stations of Liangqian, Dayan, Kongtou, Fancuo and Gaotang in Jinxi River Basin as an example, the rainfall-runoff process simulation is carried out to analyze the coupling relationship between the storage capacity of small hydropower group and its power generation and inflow flow, and the optimal output and adjustable output interval of small hydropower group in the basin are optimized. The results show that after optimization, the adjustable capacity interval is increased, the upper limit of the adjustment capacity is increased by 12.1 %, and the actual output is increased by 14.6 % compared with that before optimization. The optimized output mode can support the power grid to adjust the output of small hydropower in a larger interval, which can provide decision support for the power grid dispatching department to tap the flexibility value of small hydropower resources and give full play to its peak regulation and consumption capacity.