CO2 dissolution into an aqueous phase and water evaporation into a gaseous phase takes place during CO2 injection into an oil reservoir. This study aims to evaluate the phase behaviors of the oil-gas-water system in the displacement of crude oil by CO2. The composition of the JL oilfield in the northeast of China is taken as an example. The flash calculation of the oil-gas-water system was performed, based on the method presented by Li and Nghiem. The research results show that CO2 dissolution in the aqueous phase declines as the NaCl concentration in formation water rises. CO2 injection is beneficial for the evaporation of formation water. The NaCl concentration in formation water has little effect on water evaporation and dissolved-gas escape. When the injection-gas mole fraction of CO2 is 0.5, CO2 injection can reverse the phase behavior of the petroleum mixture and the oil-gas system is converted to a pure gas-condensate system. For CO2 injection, water vapor has little effect on the miscibility of multiple contacts, but can reduce the miscibility of the first contact.