Abstract: The solving method of water quantity and water quality model in channel networks based on the method named ‘channel-junction-channel' behaves high efficient.But,so far,there is not a good method being relative to above water quality model to calculate water environmental capacity in channel networks.In this paper,through carefully analyzing the proceeding of above water quality model in channel networks and bringing forward a new concept named ‘ermissive input pollution of junctions',a method named ‘jnnction-hannel-junction' which is relative to above water quality model and can be used to calculate unsteady water environmental capacity in a large river networks is developed and established,and is used in Taihu river networks successfully.This method can be used to the pollution analysis and water quality planning in river net.