The ternary phase diagrams of diesel oil, water, cosurfactant and surfactant system were plotted. The effect of various parameters, such as HydrophileLipophile Balance( HLB) value, the mass ratio of cosurfactant to surfactant and other parameters, on the stability of emulsifying diesel oil, was studied by using the phase change of the ternary phase diagram. It was shown that the compound of different HLB value, weak base of additive, mass ratio of cosurfactant to surfactant had a great influence on the ternary phase diagram of emulsified diesel. The largest emulsion region of the ternary phase diagram was obtained when HLB value was 5.8 for the compound of Span 80 and Tween 60 and the mass ratio of cosurfactant to surfactant was 0.3 in alkaline environment. The experimental results show that the effect of emulsifying diesel oil was best during the minimum dosage of mixed emulsifier.