Abstract: The distributive feature of soil microorganism of Pinus tabulaeformis plantation forestland in Loess Plateau under different forest age has been studied. The results shows that quantitive variety of microorganism corresponding with forest age is not remarkable, but very remarkable with soil depth; the variety of bacterism quantity corresponding with soil depth abides by Quadratics Ratio model, the quantitive variety of actinomyces and fungi corresponding with soil depth fit to index function. Compared with the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest and wasteland, the quantity of soil bacterium of Pinus tabulaeformis plantation forestland is more than wasteland, but less than the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest. This turn into a basis to build mixed forest.