Abstract: The Huaxi district of Guiyang is located in the center of Guizhou Province.The soil thickness de-veloped from carbonate rocks has a wide range.The soil quality greatly changes with different soil thickness, especially the characteristic of microorganism.Based on field investigation,3 kinds of soil types,namely thin soil,middle soil and thick soil were studied in Huaxi,Guiyang.The number and biomass of microorganism under different soil thickness in karst area were studied.The results showed that the quantity of bacteria, fungi and antinomies gradually increased with the soil thickness increase.Compared with the fungi and antin-omies,bacteria occupy an absolute advantage in soil microbes.They account for 88.13%,85.71%, 87.36%,85.00% and 77.78% of soil in different soils thickness developed from limestone and dolomite re-spectively.In limestone soil,the content of microbial biomass carbon,nitrogen and phosphorus gradually in-creased from the thin soil to the thick soil,of which the increasing amount were 15.15 mg·kg-1 ,2.82 mg· kg-1 and 0.18 mg·kg-1 ,respectively.And in dolomite soil,the content of microbial carbon,nitrogen and phosphorus increased from the thin soil to the middle soil,of which the increasing amount were 5.13 mg· kg-1 ,0.39 mg·kg-1 and 0.10 mg·kg-1 ,respectively.The content of microbial nitrogen and phosphorus in the soil developed from limestone is significantly higher than that developed from dolomite under the same soil layer,of which the difference in middle soil was the largest,with a difference of 0.90 mg· kg-1 and 0.21 mg·kg-1 respectively.In comparison,this content of microbial biomass carbon in the soil of limestone is higher than that in the dolomite soil,which is 17.31 mg·kg-1 high.The result of correlation analysis showed that the pH value had a significant effect on the microbial biomass carbon and phosphorus (p <0.05),the total phosphorus content has a significant effect on microbial biomass phosphorus(p <0.01).