A model for the alkaline grassland ecosystems, MAGE, was applied to plant communities dominated by three species. Field observations on two communities dominated respectively by Puccinellia tenuiflora and Suaeda corniculata were used to parameterize the model for multiple species interaction. The model behaves reasonably in following the seasonal variations of water content, soluble sodium cation and calcium cation in surface soil, as well as biomass of the plant communities. Simulations were run to investigate the effects of ground water quality, ground water table depth, maximum noncapillary porosity in surface soil and harvest intensity, on ecosystem dynamics. The results indicated that ground water sodium concentration and ground water table depth had primary control on soil alkalization and vegetation status. The improvement of soil conditions by vegetation is limited to an extent with moderate ground water depth and sodium concentration. Noncapillary pores are critical for vegetation to affect the soil alkalization/dealkalization process, but the effect of noncapillary pores tends to saturate when maximum noncapillary porosity is greater than 0.1.