In order to optimize the design of gas injection process and improve the oil recovery，a hydrocarbon phase behavior model was developed for flash calculation and phase stability analysis according to the theory of equation of state and miscible oil displacement. Under the setting experimental temperature and pressure，the composition change of injected gas and reservoir oil in the multiple-contact mixing process was simulated using designed composition of oil and gas. The behavior of the phase composition change during this process was discussed by use of pseudo-ternary phase diagram to determine if miscible displacement could be achieved. The simulation results showed that vaporizing-gas drive mechanism dominates if lean-gas is used as injected gas，and the vaporizing-gas drive is unable to achieve miscible displacement for the oil sample；if rich-gas is used as injected gas，the miscible process is not dominated by a single driving mechanism（vaporizing or condensing）. The simulated results indicated a combined vaporizing/condensing mechanism to explain the phase behavior of two immiscible reservoir fluids in the transition zone. The developed model and modules are able to be applied in any oil and gas system at miscible conditions，and the calculation results cast light on production.