Colonization density of Mesorhizobium huakuii JSSAI6L marked with luxAB genes on the root of Astragalus sinicus reached to the maximum (7.88 log cfu/g root) in rhizobox-soil microcosm 2 days after seed planted, then reduced and kept in a stable level for a long time. After 58 days, it rose up a little, and also the strain could dispersed to the place of 22cm from seed to root tip. The colonization dynamics in pot-soil microcosm is similar with those in rhizobox-soil microcosm. Under the field conditions, however, it reached to the maximum (7.03 log cfu/g root) 30 days after seed planted, afterward declined to the minimum (5.24 log cfu/g root) 90 days later, then began to go up to 7.89 log cfu/g root 20 days after plant harvested. Inoculation of JSSAI6L strain could increase the plant biomass of Astragalus sinicus in a large scale.