The CO2 displacement is one of the gasflooding Enhanced Oil Recovery (EOR) methods. The application from volatile oil to black oil is popular mainly because CO2 requires a relatively low miscibility pressure, which is suitable to most reservoir conditions. However, CO2 always contains some impurity, such as CH4, H2S and N2, leading to the change of phase behavior and flooding efficiency. Whether the gasflooding achieves successfully miscible displacement depends on the reservoir pressure and temperature, injected solvent and crude oil compositions. So three different types of oil samples from the real field are selected and mixtures of CH4, H2S and N2 with various CO2 concentrations as the solvent are considered. After a series of experimental data are excellently matched, three nine-pseudocomponent models are generated based on the thermodynamic Equation-of-State (EoS), which are capable of accurately predicting the complicated phase behavior. Three common tools of pressure–temperature (P–T), pressure–composition (P–X) and pseudoternary diagrams are used to display and analyze the alteration of phase behavior and types of displacement mechanism. Simulation results show that H2S is favorable to attain miscibility while CH4 and N2 are adverse, and the former can reduce the Multiple Contact Miscibility (MCM) pressure by the maximum level of 1.675 MPa per 0.1 mol. In addition, the phase envelope of the mixtures CO2/H2S displacing the reservoir oil on the pseudoternary diagram behaves a triangle shape, indicating the condensing-dominated process. While most phase envelopes of CO2/CH4 and CO2/N2 exhibit the trump and bell shapes, revealing the MCM of vaporization.