To improve demulsifying property, the original biodemulsifier-producing bacterium, Bacillus mojavensis XH1, was mutated and bred by the combined treatment method using UV irradiation and nitrosoguanidine (NTG). A high efficient biodemulsifier-producing mutant XN5 with the better stability was obtained after continuous passages. The optimal cultural conditions were as follows: the mixture of 10 g／L glucose and 4 ％ liquid paraffin is the carbon source, the mixture of 40 g／L NH₄Cl and 10 g／L yeast extract is nitrogen source, pH=65, the rotating rate is 140 r／min and the incubated time is 24 h. The 12 h and 24 h demulsification ratios of the mutant XN5 were 9417％ and 9867％, which were 6481％ and 1512％ higher than that of the original strain XH1, respectively.