Abstract: The organic fertilizer A. sinicus has a strong nitrogen fixing ability. The effect of returning A. sinicus to tobacco culti-vated soils on microbial population, activity and enzyme activity was investigated with the aim of enhancing sustainable agro-devel-opment. The treatments were set at 15 000 kg·hm^-2, 22 500 kg·hm^-2, 30 000kg·hm^-2 A. sinicus, as well as NPK-reduced 22 500kg·hm^-2 A. sinicus, in which NPK amount in A. sinicus was reduced from fertilizer, with non A. sinicus treatment as the control. The results show that aerobic bacteria population rapidly increases in the early growth stage of tobacco, followed by a sluggish increase in the middle stages, with the various population trend in the late growth stages under different treatments. During the early tobacco growth stage, soil aerobic bacteria population is increased under NPK-reduced 22 500 kg·hm^-2 A. sinicus treatment compared to the CK. Actinomycete population slowly decreases in the early growth stage, minimally increases in the middle stage and then decreases in the late stage of growth under different treatments. Actinomycete population in soils with A. sinicus is higher than in the CK at the early growth stage, and is lower under NPK-reduced 22 500 kg·hm^-2 A. sinicus treatment during the whole growth stage. Fungi population increases rapidly in the early and middle growth stages, and decreases afterwards. A little A. sinicus stimulates fungus growth. Soil microbial activity decreases in the early stages, stabilizes in the middle stages, and gradually increases, reaching its peak in the late growth stages. Microbial activity is significantly higher in soil with A. sinicus than in CK soil in tobacco mature stage. It increases with increasing A. sinicus amount, but decreases in soils with NPK-reduced 22 500 kg·hm^-2 A. sinicus. Cellulase activity increases before blossom stage and decreases afterwards. Cellulase activity in soils with A. sinicus is higher than in soils without A. sinicus in later growth stage. Initially, urease activity sharply declines, then increases slightly, but again sharply increases during the late stage. Protease activity decreases in the early stage, and increases slightly in the late stage of growth. Protease activity increases with increasing A. sinicus in the soil. Alterations in soil catalase activity are not severe in the entire tobacco growth season. Based on the analysis and evaluation of the quantity and activity of soil microbes, the optimal application rate of A. sinicus is 22 500 kg·hm^-2.