The distributive feature of soil microorganism of Robinia pseudoacacia L. plantation forestland in Loess Plateau under different forest age has been studied. The results shows that quantitive variety of bacterism corresponding with forest age is not remarkable, but actinomyces is remarkable; Quantitive variety of bacterism, actinomyces and fungi corresponding with soil depth is very outstanding; 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.