A set of experiment apparatus was established to measure vapor-liquid equilibrium data under intermediate pressure. The data of vapor-liquid equilibrium for DMF/C4 and DMF/C4 with salt were measured respectively. The experimental results show that DMF with salt had higher relative volatility of C4 and was better than DMF for separating C4. On the basis of DMF/C4 model, the system of DMF with salt and C4 was correlated according to deduced mathematic expression. The correlated results agree well with experimental data.