Abstract: Field experiments were conducted to investigate the changes of microbial population and soil nitrogen in yellow soil. Results showed that the quantities of tobacco rhizosphere bacteria increased sharply in vigorous growth stage and decreased sharply in mature stage, while the quantities of rhizosphere fungi and actinomycetes increased sharply in mature stage；the rates of mineralization increased from April to August and reached the maximum in August with 28.25 μg/g?d, but declined after August. Nitrification changed with soil moisture and basic medium, and the peak rate appeared in July with 29.95 μg/g?d. No difference of changes of microbial biomass C, N, and the ratio of Cmic to Nmic appeared in nitrogen application treatment and the control.