It is very important for sustainable utilization of grassland to study the impact of climate change on grassland productivity based on grassland ecosystem model.In this paper,Shiqu(32.98°N,98.10°E) and Ruoergai(33.58°N,102.97°E)were selected as representative sites in Northwest plateau of Sichuan province.The long-term observation data of grassland biomass from the livestock meteorological experiment stations were used to calibrate and validate the grassland ecosystem model(CENTURY).The results showed that there was a close consistency between the simulated and measured above-ground biomass at Shiqu and Ruoergai.The determination coefficients were 0.85 and 0.45 (P<0.05).Relative mean square roots were 5.4 g·m~(-2) and 122.4 g·m~(-2).The CENTURY model could be used to simulate pasture above-ground biomass and thereafter study the impact of climate change on grass productivity in Northwest plateau of Sichuan province.The changes of grassland above-ground biomass at Shiqu and Ruoergai were simulated during the past 57 years(1961-2017)based on CENTURY model.The simulation results displayed that the grassland above-ground biomass during the past 57 years increased significantly at Shiqu and Ruoergai,and the increasing rates were 5.4 g·m~(-2)·(10 a)~(-1) and 17.2 g·m~(-2)·(10 a)~(-1),respectively.