CO2 huff and puff is an effective EOR technology for tight reservoirs. However, the mass transfer between CO2 and oil makes the reservoir fluid phase behavior very complex and affects the reservoir recovery. Through the visual injection experiment, phase equilibrium calculation model and thermodynamic parameter entropy, this paper researches the change characteristics of oil phase behavior during CO2 injection. In this paper the oil phase behavior is divided into four stages and reveal the mechanism of oil phase behavior changes. The results show that in the first stage of CO2 injection, CO2 dissolves in the oil and the volume of oil expands. In the second stage, CO2 dissolves, replaces and extracts the components of the oil, forming the third phase oil-gas transition zone, so that the volume of the oil decreases rapidly. In the third stage, the mass transfer of oil-gas components slows down, and CO2 extraction effect becomes the main factor, so the oil-gas transition zone almost disappears, and the volume of the oil decreases slowly. In the fourth stage, the mass transfer of oil and CO2 stops, and due to the compression effect the volume of oil decreases linearly. Because of the effects of extraction and compression, excessive injection of CO2 is not conducive to enhanced EOR. These results can furnish an in-depth understanding of the complicated mechanisms of the CO2 huff and puff in unconventional reservoirs.