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CO2 diffusion can promote the expansion and reduce viscosity of crude oil, which plays an important role in improving the effect of CO2 flooding. However, the effect of water slug on mass transfer and diffusion of CO2 in water-crude oil system is not considered in the current study in CO2 water-alternating-gas flooding. To solve this problem, the diffusion experiments of CO2 in water-crude oil system were carried out. The dynamic compression factor of CO2 considering supercritical state was proposed, and a new model for CO2 diffusion coefficient calculation was established. The diffusion law of CO2 in water-crude oil system was studied also, and the influence of water slug, different starting time and convection on the diffusion law of CO2 in water-crude oil system were analyzed. The results show that the revised diffusion coefficients of CO2 in crude oil and formation water are 1.17×10-9 m2 /s and 0.44×10-9 m2 /s respectively. The diffusion of CO2 in the water-crude oil system can be divided into crude oil diffusion and water slug diffusion. In the crude oil diffusion, at the initial stage of molecular diffusion, it is affected by convection in a short time, resulting in a high pressure drop rate. After that it is mainly affected by concentration diffusion, resulting in a slow pressure drop rate. In water slug, the molecular diffusion is not affected by convection, the difference of CO2 concentration is smaller, and the solubility of CO2 in water is smaller than that in crude oil. So the pressure drops slowly and the pressure change rate becomes smaller. At different starting time, the pressure drop rate is different and the diffusion mass transfer rate of CO2 is different accordingly. To eliminate the influence of convection on the diffusion and mass transfer of CO2 in the initial diffusion stage, a reasonable start time should be selected.