Investigation of seven soil faunas of forest ecosystem in northern Da Hinggan Mountains respectively in June, August and October of 2004 revealed that their total number was 6729, that is, 67 categories, which belonged to 51 families, 18 orders, 7 classes and 4 phyla. The dominant group is Enchytraeidae, and its individual number takes up 58.5%. The total number of common types is about 10 categories, they are Formicidae, Lithobiomorpha etc. , accounting for 35.9%, and the number of rare groups is 56 categories. The categories of soil animals in different forest communities differ greatly, the more favorable condition the habitats is, the greater the number of individuals is, and the more abundant the categories are. The distribution law of individual, group quantity and biomass is not entirely consistent, which showed complexity of community composition, that is, the distribution of individual number is Ⅲ＞Ⅱ＞Ⅴ＞Ⅵ＞Ⅳ＞Ⅰ＞Ⅶ, the distribution of categories is Ⅳ＞Ⅲ＞Ⅱ＞Ⅴ＞Ⅵ,Ⅶ＞Ⅰ, and the distribution of biomass is Ⅲ＞Ⅱ＞Ⅳ＞Ⅶ＞Ⅴ＞Ⅰ＞Ⅵ. Furthermore, the proportion of each category of soil animals in total biomass differs greatly, and the biomass of Enchytraeidae and Lumbricidae is in absolute dominant position, accounting for 34.8% and 34.4% respectively, then the insect and its larva account for 22.8%, and the proportion of other categories is less than 8%. However, the dominant group of biomass is Lumbricidae in the forest of temperate zone, about 60% to 70% of the total biomass, which differs apparently from the biomass of soil animals in the forest of cool temperate zone; The vertical distribution of individuals and types is in the order of: 0~5cm＞litter layer＞5~10cm. The order of soil animals’ diversity index isⅠ＞Ⅳ＞Ⅶ＞Ⅴ＞Ⅱ＞Ⅲ＞Ⅵ. Evenness and richness index is positively correlated with diversity index, and is negatively correlated with the dominance index. What’s more, individual number, category, biomass and diversity index of soil animals vary with season, in other words, the dynamic change of the total number and category with season is in the order of October > August> June. The dynamic change of the total biomass with season is October> June> August. In addition, the dynamic change of diversity index with season is August > June >October.