In order to obtain dodecyl dimethyl betaine（BS-12）microemulsion system with good oil displacement performance， the effect of NaCl and n-butanol dosage on the phase change of BS-12/n-butanol/n-hexane/NaCl microemulsion was studied through Winsor phase diagram method，and the influence of phase volume change on the solubilization and interfacial tension of microemulsion was also studied. The oil displacement performance of BS-12 microemulsion system prepared according to the optimum formula was evaluated. The results showed that NaCl and n-butanol had great influence on the phase state of microemulsion. The phase of microemulsion had undergone the transition from Winsor I to Winsor III and Winsor II with increasing addition of NaCl and n-butanol. The Winsor III（middle-phase）microemulsion（1 ∶1 oil-water ratio）formed by 4% BS-12， 9.3% n-butanol and 3.8% NaCl had the strongest solubilization ability， the lowest interfacial tension and the largest oil displacementpotential. The results of indoor core displacement showed that the middle-phase microemulsion could reduced water content by 9.4% on the basis of water flooding and increased recovery rate by 8.34% ， indicating good effect of enhancing oil recovery and reducing water cut.