Fractured volcanic reservoirs are more challenging to construct than conventional reservoirs because of their complicated physical characteristics and naturally occurring fractures. Gas flooding, water flooding, gas injection, and cyclic steam stimulation are the primary exploitation techniques. It has been demonstrated that using CO2 injection to construct volcanic reservoirs works well. Diffusion, viscosity reduction in crude oil, and extraction of light hydrocarbons are the principal mechanisms of its exploitation. Determining the diffusion properties of CO2 in fractured volcanic reservoirs is therefore crucial to understanding the mechanism and impact of its utilization. In this work, CO2 diffusion experiments in saturated oil cores were carried out using an independently built high-temperature and high-pressure diffusion experiment equipment. The diffusion coefficient of CO2 was computed using experimental data, and a mathematical model for the diffusion of CO2 in saturated oil cores was developed. Analyses were conducted on the CO2 diffusion law under various pore structure. The findings demonstrated that the CO2 diffusion in saturated oil cores went through stages of fast, slow, and equilibrium diffusion. The rock's permeability and pore feature significantly affected the CO2's diffusion. A fracture might serve as a functional link between separate pores in the rock. The diffusion coefficient of cores without fractures was 10–10 m2/s, but the diffusion coefficient of fractured conditions was in the range of 10–9 m2/s. Permeability and the size of the diffusion coefficient had a positive correlation. Furthermore, the diffusion of CO2 was also influenced in part by diffusion radius and pressure.