Abstract: In this paper, high performance liquid chromatography ( HPLC) was used to determine the degradation rate of fomesafen under different conditions. The optimum degradation condition of fomesafen was obtained by Aspergillus flavus ( TZ1985) .The optimum conditions for the fomesafen degradation without adjusting pH condition were found as follows: concentration of fomesafen 10mg /L, inoculum volume of bacteria suspension 3%, temperature 25℃, shaking for 5d and the degradation rate was 92.5%.