The quantitative correlations between the soil fertility factors and microbial biomass were studied by the analysis of the soil fertility and microbial biomass contents collected from Huili tobacco growing areas of Sichuan Province. Polyfactorial regression analysis showed that different kinds of soil fertility factors could affect the contents of different kinds of microbial biomass. There were no significant correlation between pH and the content of bacteria in vigorous period, but the other four soil fertility factors were correlated with it. The available nitrogen was positively correlated with the content of bacteria in maturity period. Partial correlation and path analysis suggested that the interaction effects of available phosphorus and pH were positively correlated with the content of fungi in group pieces, also the significant effects of organic matter and pH were discovered. There were obvious interaction effects of available nitrogen and fungi in maturity period. The results also showed that actinomycetes had a correlation with available phosphorus, organic matter and pH in three different periods, and the relationship between soil fertility factors and microbial biomass was different.