A simulated warming experiment using infrared radiators was conducted to study the effects of climate warming on phenology and grass yield of alfalfa,so as to provide the theoretical basis for grass yield prediction in the future climate warming conditions. The results indicated that warming increased mean soil temperature by 0.7 ℃,and reduced mean soil moisture by 11.3%,compared with the control (normal temperature treatment). Warming significantly accelerated alfalfa seedling,branching,budding,early flowering,and flowering stages by 2.0,5.8,8.7,9.0 and 3.3 days,respectively. Warming significantly decreased leaf net photosynthetic rate by 13.2%,and reduced grass yield by 13.8%. Moreover,warming significantly decreased root nitrogen and carbon content by 7.4% and 11.8%,respectively,and reduced soil nitrogen content by 4.5%. The correlation and regression analysis showed that the time when alfalfa reached flower stage and grass yield of alfalfa showed significantly linear increase trends with the increase of soil nitrogen content,respectively. This indicated that flowering stage of alfalfa may advance,and grass yield would reduce with the decrease of soil nitrogen content under future climate warming scenarios.