Ecotoxicology of sulfamethazine on soil microbial activity and its degradation and transfer in the soil were investigated. Results show that 1 mg kg^-1 concentration of sulfamethazine in soil resulted in an apparent decrease in populations of bacteria and fungi (p < 0.05). Moreover, sulfamethazine was observed hardly degrading in soil, with a half life of 223.9 d and 102.4 d in sterilized and unsterilized soil respectively. And the thin layer chromatogram and leaching test indicated that sulfamethazine was liable to transfer and get leached, with R_f being more than 0.7, and its transfer and leaching rates were subject to the impact of pH and increased in acidic and alkaline developing agents, suggesting that sulfamethazine, as veterinary antibiotic, is a new environmental contaminant strong in ecotoxicology and mobility, low in degradation rate in soil, thus posing a potential pollut ion risk to water bodies.