The sluice construction has a blocking effect on river, which changes the flow pattern, the transport of pollutants and the temporal and spatial distributions of water quality, hence may have an impact on the water environmental capacity. In this study, the urban section of the Lhasa River is taken as the research case, and the influence of sluice construction on hydrodynamic conditions has been studied. On the basis of hydrodynamic and water quality simulations of two-dimensional model, the water environmental capacity of COD before and after sluice construction is calculated and analyzed, and the influence of continuous sluice construction on river water environmental capacity is discussed. The results show that there are two main aspects of the influence of sluice construction on water environmental capacity. First, once sluice stores water, the water body changes from flowing to relatively static state, the flow velocity in front of sluice slows down, the water depth increases; these affect the diffusion of pollutants, reduce the degradation coefficient of pollutants, and then affect the degradation process of pollutants. Second, sluice storage makes pollutants stay in the reservoir area, so that the retention time of pollutants will be increased, which leads to the increase of degradation of pollutants. After the construction of the Lhasa River sluices, due to the influence of sluices water storage, the velocity of pollutants is slowed down, and the diffusion of pollutants to the central part of the river is hindered, which allows pollutants to easily accumulate on the sewage-draining side. Comparing with the natural river, the average flow velocity in the sluice storage area decreased from 0.34 m/s to 0.10 m/s, the average water depth increased from 0.6 m to 2.0 m, and the COD degradation coefficient decreased from 0.12 d^-1 to 0.04 d^-1 (about 67% decrease), and the pollutant retention time increased by about three times. The calculation results show that, one sluice reduces the COD capacity by about 2.2%, two 9.9%, and three 12.3%. Overall, the continuous sluices have certain impacts on the river water environmental capacity. The more sluices, the greater the impacts on the water environmental capacity.