Abstract: A nonlinear dynamic model of water used in agriculture-water resources-city population is established and also the ecological effect of water used in agriculture and its stability of this model are stated in the paper.The results show that the excess water used in agriculture may lead the suburb area to urbanism or the town to ruralism;the stable city population is linear to the total crops yield;with a very low irrigation rate,more water used in agriculture can not lead to the increase of crops yield,but the decrease of the crops yield and reducing of the city scale;in order to increase more agriculture～used water to increase the corps yield and to expand the city scale,JP2the only way is to improve the agricultural irrigation rate.