Abstract: To investigate the changes of soil microbial characteristics in Karst mountain areas caused by the transformation from conventional cultivation into organic cultivation, an experiment was carried out in tobacco fields on which organic cultivation had been conducted for three consecutive years in Jinsha County, Guizhou Province by taking conventional cultivation as the control. The basic soil properties,bacterial and fungal community structure and key influencing factors were studied. The results showedthat: comparing with conventional cultivation, the contents of soil organic matter and microbial biomass carbon significantly increased, the contents of soil nutrients(total nitrogen, total potassium, available phosphorus,available potassium, etc) significantly decreased(p<0.05), and the amount of beneficial microorganism groups increased as a result of organic cultivation. The richness and diversity of bacteria were higher than those of fungi were common to both cultural practices. The diversities and microbial communities at phylum level were not obviously different between conventional and organic cultural practices. Bacterial communities dominated by Proteobacteria, Actinobacteria and Acidobacteria(accounted for 54.2%-62.0%), and fungal communities dominated by Ascomycota(accounted for 74.4%). The microbial communities in Karst mountain areas were mainly influenced by the organic matter, nutrient and bulk density in soil.