It is a common phenomenon that the balance position of the piston is offset in the thermal machinery composed of high-frequency reciprocating piston with clearance seal.For linear compressor and free-piston Stirling generator,the piston offset phenomenon will lead to the fracture of the plate spring,the decrease of output efficiency and the impact between piston and cylinder.In order to study the internal mechanism of the piston offset character under clearance seal and its affecting factors,the flow and leakage model of the clearance was established,and the accuracy of the model was verified through numerical simulation.Based on the sealing structure of the hundred watt free-piston Stirling generator,a thermodynamic and dynamic model was established,and the character of the piston offset caused by the clearance size and the external force was analyzed through the model.The results show that the reason of the piston offset phenomenon is that the net leakage of clearance is greater than zero in one cycle.The piston offset increases with the increase of the clearance size and external force under certain conditions.