To analyze the spatiotemporal characteristics of the fluoride water environmental capacity in the main channels of the lower Ganjiang River network, the T-Copula function was introduced to construct an uncertainty water environmental capacity calculation framework based on the flow rate-water quality joint probability density distribution. Taking fluoride as the target pollutant, the distribution characteristics of the fluoride water environmental capacity in the main channels of the lower Ganjiang River network during different periods in 2024 were analyzed, and a sensitivity analysis was conducted based on the modified one-variable-at-a-time (OAT) method. The results indicate that all main channels possess a certain water environmental capacity throughout the year, although some channels face a potential risk of exceeding this capacity. The highest and lowest expected values of water environmental capacity were observed in the mainstream section of the lower Ganjiang River network and the northern branch’s fishery water use zone, respectively. The expected values of the water environmental capacity in the river network during different periods exhibit a certain fluctuating pattern. The seasonal variation in river flow is identified as the key driving factor causing the dynamic changes in the fluoride water environmental capacity in the lower river network.