Decay factor is very important for numerical simulation of the assimilative capacity for a river, because it determines the accuracy and reliability of the water quality model. Based on Mike21, a two dimensional water quality model was established to analyze the assimilative capacity of the typical river breach between the Three Gorges Dam and the Gezhouba Dam. Ammonia nitrogen was chosen as the typical pollutant. Sensitivity analysis was performed by changing the discharge, background concentration of the pollutant and decay factor. The simulation results show that: Compared with the background concentration, the change of discharge has a more obvious effect on the pollutant diffusion process. The diffusion range of pollutant increased with the increase of discharge and decreased with the increase of background concentration of the pollutant. Increasing the decay coefficient will reduce the range of pollutant diffusion, and the range of decay coefficient will be larger when the discharge is high and the background concentration of the pollutant is low, which makes the range of influence maximum.