Abstract: In this paper,the carrying capacity of urban water resources and water environment are discussed in several aspects,such as the concept,the connotative meanings and the computation model.A computation method and the model relevant to urban water carrying capacity are put forward which are called "the Control Object Inversion Model"(COIM).including the equation of total drainage from urban area,the equations of annual runoff volume and concentration of contaminant at the control cross section,and the control objective equation,and the computation model of carrying capacity of urban water environment.With a case study of Zhengzhou city,the application of the model is presented and some countermeasures against water environment deterioration are proposed.