Abstract: Soil acidity is the main factor affecting plant production all over the world.Alfalfa cultivation in southern China also confronts this problem.In order to screen suitable alfalfa varieties and rhizobium for acid soil,5 months pod experiment was carried out to analyze the nodulation ability of different varieties and rhizobium in acid purple soil in three gorges reservoir.The purple soil was selected from hilly arable land in Beibei,Chongqing.Soil acidity was pH 5.45.Alfalfa was not cultivated in the land for the past ten years.Six Medicago sativa varieties used in trial were Zhongmu 1(A),Gannong 1(B),Gannong 3(C),Sandtti(D),Eureka(E) and Defi(F).The results showed that the root and shoot biomass of alfalfa were significantly increased by inoculating Sinorhizobium melilotii.Different rhizobia had significant difference in nodulation ability and their effects on plants,and different alfalfa varieties also had different nodulation abilities.The rhizobium with better nodulation ability increased the content of nitrogen,phosphorus and calcium in shoot,while decreased the content of potassium and aluminum in shoot.The content of nitrogen,phosphorus,potassium,calcium and magnesium in shoot were significantly higher than those in root,while the content of aluminum in root with inoculating rhizobia was significantly higher than that in shoot.Nodule weight and number were significantly correlated with shoot biomass and nitrogen content in shoot,while root aluminum content was negatively correlated with shoot biomass,root biomass,nodule weight,nodule number and nitrogen content.It indicated that good nodulation ability increased biological nitrogen fixation and growth of alfalfa.But over absorption of aluminum by root in acid purple soil constrained the nodulation and nutrient absorption of alfalfa,and thus influence the adaptation of alfalfa in acid soil.It was suggested in the paper that more alfalfa varieties and efficient rhizobia should be screened before the alfalfa were successfully planted in acid soil.