Carbon dioxide in a supercritical form can be stored in deep unmineable coal seams in great quantities than in the free state. However, other components of gases in coal seams can affect the storage conditions of the supercritical CO2. Using the critical opalescence of carbon dioxide as a critical point criterion, with the experimental temperature 10~40 °C, and pressure 10 MPa, the critical points of ten groups of CH4-CO2 binary system and CH4-CO2+coal sample ternary system were measured with a high pressure visualization device with a sapphire window. The effects of different contents of CH4 on the supercritical CO2 were studied. The results show that, the critical temperature and critical pressure of the CH4-CO2 binary system varies with CH4 content. When the CH4 component concentration is less than 10%, the critical temperature range of the CO2-CH4 binary system is between the critical temperatures of the two components, it is lower than that of pure CO2 and higher than that of pure CH4. The critical pressure increases with the increase of CH4. After adding coal, CO2-CH4 binary mixture critical opalescence phenomenon still occurs, but the critical temperature is slightly higher than that of the corresponding CH4-CO2 binary system, while the critical pressure is lower than that of the binary system.