To investigate the microstructural characteristics of the filling structure in a regenerator, based on the fractal theory of porous media, the mercury intrusion method was used to study the pore distribution and fractal dimension of the regenerator. The regenerator is a key component of a miniature Stirling cooler. The regenerator prepared by filling stainless steel mesh or stainless steel felt is a typical porous medium. The microstructure of the regenerator was tested using the mercury intrusion method, and the pore distribution range inside the regenerator was obtained. Combined with the basic theory of fractal analysis of porous media, the fractal dimension of the regenerator is calculated, which shows that the regenerator has fractal characteristics, and the fractal dimension interval of the regenerator can be obtained.