Two inoculation methods including seed surface and root inoculations were used to study the effects of inoculation quantity of viable rhizobia Mesorhizobium huakuii on the nodule formation and nitrogen fixation in Astragalus sinicus. The minimum inoculation quantity of rhizobia required for the nodule formation and nitrogen fixation in A. sinicus was determined with a sterile sand-pot experiment system using liquid bacterial agents as inoculants. The results showed that the plant leaves started to turn from yellow to green， and the fresh weight and plant height aboveground significantly increased compared to that in treatments with lower inoculation quantity when the rhizobia concentration inoculated on the seed surface reached 102 CFU/seed. Meanwhile， nitrogen-fixing nodules developed on the roots of the A. sinicus plants， having a positive effect on promoting growth. When the plant root was inoculated， a single A. sinicus plant required only an inoculation quantity of 10¹ CFU/plant to initiate the nodule formation and nitrogen fixation. When the quantity of rhizobia inoculated on a single A. sinicus plant reached 10³ CFU， its nodule formation and nitrogen fixation， biomass， and plant growth was significantly better than that in treatments with low inoculation quantity. Further increasing the inoculation quantity stabilized the symbiotic nitrogen fixation. 10³ CFU/plant was determined to be the minimum inoculation quantity of M. huakuii rhizobia on A. sinicus to fully ensure the nodule formation and nitrogen fixation of rhizobia inoculants after application in the field， and take the economic effectiveness of inoculant application and the complexity of soil conditions into account.