In order to have a deep understanding of the kinematical characteristics of the piston of free-piston Stirling engine and finding the way to increase the efficiency, the effects of gas force are analyzed. During the course of the experiment a new calculation method for natural frequency of piston is proposed. In this paper, the gas force acting on the piston is analyzed in the case of pressure leading or lagging displacement of piston. And the acting mechanism of gas force is stated by means of rotation vector decomposition. The results show that in the case of the pressure wave in advance or delaying the displacement of piston less than 90°, the gas spring and the mechanical spring are parallelarranged, so the natural frequency of the system increases.The gas force is regarded as gas inertia force, so the natural frequency of the piston decreases in the case of the pressure wave in advance or delaying the displacement of piston greater than 90°but less than 180°.The natural frequency of the piston increases with the temperature of the heat source. The above results are verified by an experimental test on a free-piston Stirling engine designed by our laboratory. Through the study, the free-piston Stirling engine has been started-up successfully.