In order to avoid improper selection of model to be exaggerated or reduced water environmental carrying capacity,and consequently to be unfavourable to the situation of water resources protection, this paper is based on water quality model of one-dimensional steady-state conditions to deduce the three models of the water environmental capacity that is the first section control method, the standard model and the European Union model．Taking Weihe river main stream of water function in shaanxi section for example,Calculating the COD and ammonia nitrogen water environmental capacity of the different functional areas in different designs hydrological conditions,And the different model calculation results were analyzed,Maximum and minimum range of the carrying capacity are proposed as the result．The results show that this method is more reliable, and better service for decision makers．