CO2 flooding crude oil may generate abundant foams in oil field separators after depressurization. A large amount of foams occupies most of the space in a separator and spends much time to burst, which, in turn, reduces the separation efficiency of the separator. Moreover, the incomplete gas-liquid separation will cause corrosion in the downstream pipeline, and damage both the pump and the compressor. Based on the self-development crude oil depressurization foaming device and the CO2 flooding crude oil, this paper studies the evolution of foam and analyzes the function mechanism of asphaltene and wax crystal on defoaming and drainage of crude oil quantitatively. The experimental results show a law that the rate of defoaming and drainage increase at the initial stage and then decrease, which is available for different crude oils. So, the paper proposes a kinetics model of the defoaming and drainage based on the three-degree polynomial function innovatively, which not only describes the evolution of foam volume, but also indicates the change of foam drainage rate. Meanwhile, the experiments found that asphaltene promoted the stability of foams by being adsorbed on foam film in the process of liquid drainage. Wax crystal reduced the fluidity of drainage, which decreased the defoaming and discharging rate. But the wax crystal with specific structures reduced the stability of foams. It is significant to explore the foaming and defoaming mechanism of crude oil for providing references for the improvement of separator efficiency and the development of separation technology.