To investigate the effects of replanting poplar on soil microorganism, four soil samples were collected from Chenwei poplar forestry center of Sihong county, Jiangsu Province. The physical and chemical properties of soil were analyzed and the concerned microorganisms in them were counted. The results showed that long-term plantating of poplar increase of pH as high as 1.5 after 14 years of plantation. The contents of N, P in soil decreased as years advanced while the contents of organic matter increased gradually. The amount of bacteria, actinomycete as well as the microbial biomass changed in accordance with that of the organic matter. On the other hand, the amount of fungi improved along the expansion of time. Concerning microbe involved in the cycling of N, the amount of denitrate bacteria was demonstrated to drop obviously as the time of planting advanced, while that of nitrobacteria increased to some extent.