Abstract: Water is a common limiting factor for plant growth in the vast grassland in the arid and semi-arid northern China. A differential equation model for the plant-water system was constructed in this research which stresses the interaction and coupling effect between vegetation and water circulation. Two quantities, soil water content and plant biomass, were selected as state variables. The objective of the model is to describe the variation of the two quantities in a typical grouth period (one year). Theory of optimal control was then applied to the differetial equation system to obtain the possible maximum harvest with the system stability as the main constraint.