The degradation coefficient of pollutant is an important parameter which can reflect the change situation of water quality,set up the water quality model and calculate the water environment capacity of river.The paper utilized segmentation anti algorithm method based on measured data to calculate the degradation coefficient of CODMn and Ammonia during water diversion of Dagu river.The water quality model were built to validate the applicability of degradation coefficient.The results indicated that the water quality model built from degradation coefficient has high accuracy. The degradation coefficient in the study can be used to research the water environment capacity and pollution perceiving capacity of Dagu river.