Based on the method of selecting samples in field, the ecological characters of small-middle size soil animals communities were investigated in a compound ecosystem of farmland, grassland and woodland in the grassland region of Songnen Plain. The results showed that the dominant groups were Hypogastruridae, Actinedida, Gamasida and Oribatida; and the common groups were Isotomidae and Jassidae. From the comparison analysis of farmland, grassland and woodland, the diversity of small-middle size soil animals communities was the highest in grassland, and the habitat in grassland was superior to the other two. The number of individuals and groups of small-middle size soil animals was decreased with increasing soil depth, and the surface gathering was obvious. But, the characteristics of vertical distribution in the three subsystems were different. Using the principal component analysis, the correlations between soil environmental factors and small-middle size soil animals communities and those between climate factors and small-middle size soil animals communities were analyzed. The results showed that soil environmental factors were the dominant factors affecting small-middle size soil animals communities.