Abstract: In order to investigate the suitable variety,timing,methods and the effects of reseeding alfalfa(Medicago sativa) in the natural grassland in northern China,we conducted an experiment about alfalfa reseeding treatments in the Hulun Buir natural grassland and studied the effects of reseeding alfalfa on the community biomass,species richness and the available N and P content in the soil.The results showed that the community biomass,species richness and the content of available N and P in the soil increased in the year of reseeding alfalfa,but there was no significant differences compared with the control.The first time reseeding (in the middle of May) was better than the second time reseeding (in the middle of June) on the community biomass,species richness and the content of available N and P in the soil.The effect of reseeding time on the community biomass was significant (P<0.05).The broadcast sowing was better than the row seeding treatment in the community biomass,species richness,the available N and P content in the soil.The effect of reseeding method on the available P content in the soil was significant (P<0.05).