The invention discloses a CO2 injecting and oil deposit displacing CO2 mixed-phase area judging method. The method comprises the steps that fine oil deposit description is conducted on a target oil deposit, and a fine oil deposit geologic model is established; numerical simulation research of the target oil deposit is carried out, and an oil layer pressure distribution characteristic plate and an oil layer CO2 saturability distribution characteristic plate are obtained; through a long thin tube displacement test, the minimum mixed-phase pressure of CO2 and crude oil of the target oil deposit is determined through measurement; and a CO2 injecting and oil deposit displacing CO2 mixed-phase area is judged. The method is clear in step and easy and convenient to apply, and a set of scientific, reasonable and reliable judgment and evaluation criteria is provided for developing research of a low-permeability (compact) oil deposit CO2 injecting and oil displacing mechanism and the CO2 injecting displacement recovery efficiency improvement technique. Whether an oil deposit belongs to mixed-phase displacement or not can be judged, and whether an area to be evaluated in the oil deposit is of the mixed-phase area type or not can also be judged. Meanwhile, the method can guide low-permeability (compact) oil deposit CO2 injecting displacement to carry out corresponding development strategy research. The method has broad application and popularization prospects and remarkable economic and social benefits.