The operating frequency is key to free piston Stirling engine. The free piston Stirling engine is different from the traditional crank link Stirling engine, that the frequency is decided by the dynamic parameter and thermodynamic parameter, such as charge pressure, moving mass, stiffness of spring, damped coefficient. So, it is not easy to calculate the operating frequency of free piston Stirling engine. In this paper a model that includes dynamic and thermodynamic methods is used to study the characteristics of frequency. The pressure is lineared to get the computational formula of frequency. The influence of main parameters to frequency is studied through the model, and the model is validated byrelative experiments. At last, the reason why the frequency is unable to increase by enhancing the stiffness of spring is analyzed. The natural frequency of spring has a big effect on the operating frequency of engine. To increase the operating frequency of engine, a new spring is designed to increase it from 35 Hz to 60 Hz.