Influenced by tides,pollutants in rivers can be transferred and changed a lot.This paper focus on how to set the reasonable sections for monitoring and to establish in-phase monitoring project that in accord with the law of water quality and quantity of the reach,so as to provide service for finding out the contamination input and getting more actual evaluation of water quality.It can be found,from the in-phase monitoring of water quality and quantity of the reach at the junction of Guangzhou and Foshan,that the sum transport of COD into this area is 288.8 t per day,and the sum transport of NH₄⁺-N is 88.8 t per day,and that using representative period method to calculate the contamination input and using individual item law to evaluate water quality are more credible and reasonable.Besides,the reason responsible for worse water quality is that contamination carrying capacity goes over environmental capacity of the reach.Intensifying surface pollution governance of the upstream areas of the reach,and establishing water quality automatic monitoring station in important water supply places to monitor real-time water quality are effective measures to ensure safety of water taking in different seasons.