Abstract: The impact of climate and environment on productivity of ecosystems is a complex process. Thorough understanding of these processes is helpful for estimation, prediction and management of productivity of ecosystems. The current coupled mode l of ecophysiological and physical transfer processes provides a useful tool for simulating response mechanism of ecosystem to climate and environment. It is possible to explore, to a certain extend, the formation process of ecosystem produ ctivity in a changing climate. AVIM used in this study is a dynamical land surface process model that involves both physical transfer processes between soil, vegetation and the atmosphere and plant ecophysiological processes. Therefore, AVIM is capable of calculation of surface fluxes of energy and water and output of carbon flux and productivity. Semiarid steppe in middle latitude is transient climateecosystem zone that is sensitive to climate change. The response mechanism of steppe productivity (Net Primary Productivity, NPP) in semiarid areas to temperature and precipitation change was simulated with the AVIM. The results showed that changes of both tempera ture and precipitation had significant influence on NPP. The increasing of preci pitation could increase NPP while the increasing of temperature could decrease NPP. The reason for this is thought that the increasing of precipitation could im prove soil water stress and therefore enhance the photosynthesis rate. Increase in temperature could increase both photosynthesis and respiration rates on the one hand, on the other hand, it also decreases soil moisture due to increased eva potranspiration and thus results in the decrease in the photosynthesis rate. Since water condition plays a key role for plant the photosynthesis in semiarid region, so the total effects of temperature increasing caused a decrease in NPP. Sensitive tests show that for semiarid steppe in China, ±2 ℃ temperature chang e could resulting in a variation of about 20% annual NPP and at least 30% above ground biomass. 50% change of precipitation could create 37% and 50% annual change of NPP and above ground biomass respectively.