In order to explore approaches to improve the nitrogen fixation ability of alfalfa rhizobia,the effects of different concentrations of exogenous NO on the growth of rhizobia and the sensitivity of different rhizobia to exogenous NO were studied by using SNP as the donor of exogenous NO to screen out the suitable concentration of exogenous NO for the growth of rhizobia and the rhizobia strains that can be combined with NO.The results showed that when SNP concentration was 0.08 mmol/L,the number of rhizobia was significantly higher than that of other treatments,and its OD value was 0.968 0.Exogenous NO promoted the growth of TA34 significantly,and the OD value increased 46.01% compared with CK,but it inhibited WL68 significantly,and the OD value decreased 71.63% compared with CK.Different strains showed different sensitivities to NO,and WL68 was the most sensitive to exogenous NO,while GL28 was not sensitive.