The efficiency of regenerator of Stirling engine influences mightily the performance of Stirling engine. The periodical oscillating flow in the regenerator and the heat transfer characteristics between fluid and regenerator are modeled in two dimensions by the dynamic mesh model of the commercial CFD software FLUENT. Moreover, the regenerator temperature field can be also simulated when the Stirling engine begins to work. The results show that the effectiveness of regenerator and loss of flow resistance decrease with the increase of the porosity, and increase with the increase of length-diameter ratio. Therefore the porosity and length-diameter ratio need to be selected according to the overall design requirements of the engine.