Abstract: Aiming at the phenomenon of the gradual increase of NH₃-N in the effluent stage of the improved alternating biological pond,the variation of the water quality with time was studied by setting different sampling locations and sampling depths. It has shown that the improved alternating biological pond had obvious denitrification phenomenon in the effluent stage of the side pond. The denitrification was basically completed within the first 2 hours, and the denitrification phenomenon became more obvious with the increase of water depth.Sludge released NH₃-N, which was the main reason for the gradual increase of NH₃-N concentration with the extension of effluent time.