In the penetration step, a carbon dioxide fluid is contacted to a sample of coffee green bean inside a chamber at a temperature of 28-50° C. and a pressure of 950-3,500 psi for 3-30 minutes to allow the carbon dioxide fluid to penetrate into the sample of coffee green bean. In a pressure relief step, the pressure in the chamber is reduced to normal pressure in a time period of 2-15 minutes, allowing the carbon dioxide fluid penetrated into the sample of coffee green bean to break down cell wall of the sample of coffee green bean to obtain a sample of coffee green bean with broken cell wall.