Abstract: To understand the biological characteristics under different soil thickness in karst area, this paper takes the soil developed from dolomite and limestone in Huaxi district of Guiyang as the research object. The method of field sampling and laboratory analysis was used. The soil enzyme activity, microbial biomass and microbial quantity were analyzed. The results showed that the invertase, urease, phosphatase, catalase and polyphenol oxidase activity reduced with the soil thickness decreasing. Except the catalase, the soil enzyme activity in thin soil was significantly lower than that of middle and thick soil.The quantity of bacteria, fungi, antinomies and microbial biomass phosphorus gradually increased with the soil thickness increasing. Compared with the fungi and antinomies, the bacteria were dominant and the proportion was 88.13%, 85.71%, 87.36%, 85.00% and 77.78% in microbial amount respectively in different soil thickness. The microbial biomass gradually reduced with the soil depth decreasing. The microbial biomass nitrogen content is significantly higher than thin soil and middle soil, its content is 3.48 mg/kg. The ratio of MBP/TP is minimum.