CO2 storage in coal seam is the important way of reducing CO2 emission into atmosphere and enhancing coal-bed methane (CBM) recovery. This paper based upon the results of CO2 storage in coal seam in theoretical research and demonstration project, analyzed the mechanism of CO2 storage in coal seam, and recognized that the process is a complex process from a single physical process to physical, chemical, and microbial effect process. The paper also summarized the fluid-solid coupling actions between H2O-CO2 and minerals in coal seam and its effect on coal reservoir characteristics. It is considered that after H2O-CO2 action, the components of coal petrography changed. At the same time, there has been marked change in coal physical properties: the porosity enhanced, mechanics property reduced, and the changed of adsorptivity and permeability need further researches. This paper can guide the prediction storage ability and assessment the safety of CO2 in coal seam.