Abstract: In order to screen out the bacteria strain that can degrade sulphonated drilling fluid efficiently, a series of studies were carried out with the sulphonated drilling fluid as the only carbon source and some soil contaminated by the sulphonated drilling fluid in Xinjiang as the bacteria source. The bacteria strain Xh8 that can degrade sulphonated drilling fluid more efficiently was screened out by means of enrichment, acclimation and separation. And according to the evaluation, it is Bacillus flexus. Then, its classification status was determined by virtue of physiological & biochemical and 16S rRNA gene sequence analysis. In addition, its degradation property was figured out through single-factor experiment and orthogonal experiment. The single-factor experiment results show that the growth of Xh8 is optimal in the condition of pH value 9.5, salt mass fraction 4%, rotation speed 150 r/min, temperature 35 ℃, inoculation amount 10%, N-source peptone, and P-source K2HPO4. The orthogonal experiment results show that the optimal COD degradation condition of sulphonated drilling fluid is pH value 9.5, salt mass fraction 4.5%, rotation speed 150 r/min and temperature 35 ℃. After being cultured for 7 days, the COD degradation rate of sulphonated drilling fluid is 22.9%. The experimental results indicate that Xh8 can be taken as the preferred bacteria strain for the degradation of sulphonated drilling fluid. The research results can provide the theoretical guidance for the further treatment of the strata contaminated by sulphonated drilling fluid.