In the clearance seal of free piston Stirling power piston,the gas leakage in the clearance will cause the change of pressure and gas quality in the working chamber,and then affect the working efficiency of Stirling machine.In order to study the dynamic influence of gas leakage on pressure during the compression cycle,three physical models of the clearance seals,the model with clearance seal length unchanged,the model with unilateral change of clearance seal length and the model with bilateral change of clearance seal length were established,and the time advance method was used to analyze and solve the influence of different types of seals on the leakage.The results show that the one-way leakage of the clearance seal is the largest at the start-up stage.With the time advance,the leakage gradually decreases and then reaches stability.The one-way leakage of the model with clearance seal length unchanged is the least compared with the other two models.Based on the model with clearance seal length unchanged,the influence of film thickness,back pressure and seal length on the leakage was analyzed,and the design of film gap and back pressure were optimized.It is found that the leakage of gas mass increases with the increase of gas film thickness and back pressure,and decreases with the increase of piston length.When the gas film gap is between 20 μm and 30 μm,the piston length is from 10 cm to 15 cm and the back pressure is from 3 MPa to 5 MPa,the leakage of the clearance seal is controlled within 3%,which is in line with the design of the power piston clearance seal.The analysis results provide a reasonable design basis for the free piston Stirling machine power piston clearance seal.