Abstract: To study the effects of the different plantations on the soil microbial biomass carbon, nitrogen, phosphorus and physicochemical characteristics, three plantations [Robinia pseudoacacia plantation(RP), Cupressus duclouxiana plantation(CD),R. pseudoacacia-C. duclouxiana mixed plantation(RC)] and a non-plantation area(CK)at the Karst regions were chosen as the research objects. The results showed that: Except the content of Total P,the contents of SOC, Total N, available N, available P and water of RC were higher than those of RP and CD. No differences were found among the soil pH of the three plantations. The soil bulk density was followed the order of CD, CK, RP, RC. The highest content of soil microbial biomass carbon was found in RC(267.33 mg/kg),followed by CD(163.34 mg/kg), the non-plantation area(125.62 mg/kg)and RP(97.24 mg/kg). The highest content of soil microbial biomass nitrogen was RC(38.45 mg/kg),followed by RP(27.70 mg/kg), CK(18.51 mg/kg)and CD(14.81 mg/kg). The highest content of soil microbial biomass phosphorus was still detected in RC,followed by CD, RP and CK.All of the results illustrated that RC could remarkably improve the soil nutrient and increase the microbial biomass carbon,nitrogen and phosphorus in the Karst regions.