Seasonal variations of soil microbial communities in surface soil (0–10 cm) of undisturbed typical black soil (the Populus davidiana natural secondary forest in Keshan farm, Heilongjiang Province of northeastern China) was studied by using the method of dilution-plate method and Biolog ECO-microplate culture. Results showed that soil microbial population was the maximum in summer, fewer in spring, and the minimum in autumn. The quantity of soil microorganism, bacteria, fungi and actinomycetes was (4.76–16.44)×106, (4.68–16.19)×106, (4.29–10.07)×104 and (3.97–14.96)×104 cfu/g dry soil, respectively. The peak value of the average well color development (AWCD) was found in summer followed by spring and autumn in consequence, varying from 0.81 to 1.21 (168 h). The microbial functional diversity was significantly higher in summer than in spring and autumn (P<0.05) and showed no significant difference in spring and autumn. For summer, Richness index was 27, Shannon-Wiener index was 3.18, Simpson index was 0.95 and McIntosh index was 8.19. The carbon sources with higher utilization efficiency or utilization intensity by soil microbes were amino acids, polymers and carboxylic acids. Above all, this result would provide more important primitive characteristics of microbial features in the degradation and restoration process of the quality of the soil habitat scientifically.