Abstract: A differential equation system model was proposed and implemented to simulate the dynamic interactions between the plants and soil variables such as water and main salt element concentrations on the alkalized grassland on songnen plain in Northeast China. The system variables in the model included above and underground biomass of major plant species, soil water content, soil organic content, and soluble and exchangeable cation concentrations of sodium and calcium in the soil. The processes considered in the model included soil cation and water movement, plant growth under different soil alkality, plant transpiration and soil evaporation, liter accumulation and decomposition, and soil organic matter accumulation and mineralization. The model emphasized the effects of underground biomass on soil moisture characteristics and soilhydraulic conductivity and successfully explained the coupling between the biomass and soil environment.