Abstract: Maxwell and Circuit Editor were used to analyze the performance of the free-piston Stirling generator under resistive and capacitive loads. The magnetic induction intensity distribution and the voltage and current at both ends were obtained for resistive and capacitive loads， respectively. The field-circuit coupling method was used to obtain the output power of the free-piston Stirling generator. It is found that the capacitance in series in the external circuit can make the generator reach circuit resonance， which can improve the performance of the free-piston Stirling generator.