Vegetation recovery is a key measure to improve ecosystems in the Loess Plateau of China. In order to understand the soil micro-organism and evolution in artificial woodland in loess hilly area of the Loess Plateau，the soil microbial biomass，microbial respiration and physico-chemical properties in Robini apseudoacacia soils were studied. In this study, eight woodland soils with different ages were used to study the evolution, and a farmland, a native forest communities（Platycladus orientailis L.）were chosen as the references. The results showed that soil quality were steadily improved on soil microbial biomass，metabolic quotient and physical and chemical properties after plantation. Soil microbial biomass C,N,P increased significantly after 10-15 years of de-farming and vegetation recovery, and a relatively stable state was kept in near matured or matured forest, then a rise appeared again at the end of mature stage. After 50 years of de-farming and vegetation recovery, soil microbial biomass C,N,P（SMBC SMBN SMBP）, respectively, are increased by 213%,201% and 83% compared with the farmland, however, they are only 50.98%,55.17%and 61.48% of that in Platycladus orientailis’s soil, respectively. Soil microbial respiration was enhanced in the early stage and then weakened in the late stage after plant restoration. This is different from the change of soil organic carbon. Metabolic quotient (qCO₂) was significantly higher in Platycladus orientailis’s soil than that in farmland at the early restoration stage, and then decreased rapidly. After 25 years of de-farming and vegetation recovery, qCO₂ is lower than that in the farmland’s, and reach the minium after 50 years which is close to Platycladus orientailis’s. A significant relationship was found between soil microbial biomass, qCO₂ and physico-chemical properties and restoration duration. The results suggested that it is possible to improve eco-environments and soil quality in the loess hilly area of Loess Plateau by artificial vegetations recovery, but a long time, maybe more than 100 years, is required to reach to the climax before vegetation destruction.