Fly ash-filled polymer foam materials are favored by the new material industry because of their light weight, high strength, environmental protection and many other superior properties. However, the uneven distribution of fly ash limits its application. Fly ash with different particle size ranges (80~150 μm, 45~80 μm, < 45 μm) and different mass fractions (4%, 8%, 12%, 16%) were introduced into the E51 epoxy resin-4,4'-diaminodiphenyl sulfone-poly (propylene carbonate) (EP-DDS-PPC) polymerization system, and fly ash filled epoxy resin foams were prepared by reaction-induced phase separation. The results show that the decrease of fly ash particle size can alleviate its deposition effect under gravity field and help to improve the mechanical properties of the material. When the fly ash with particle size <45 μm is used and the filling amount is 4%, the tensile strength of the material reaches 42.33 MPa. With the increase of fly ash content from 4% to 16%, the average pore size of the material increases, the pore density decreases, and secondary phase separation occurs. In addition, within a limited range, the incorporation of fly ash did not significantly affect the thermodynamic stability of the material, however, it significantly reduced the tensile strength of the material, which may be due to the pore structure change induced by fly ash. This study not only enriches the theoretical framework of reaction-induced phase separation involving solid particles, but also provides a solid theoretical basis and practical guidance for the green and efficient preparation of porous materials.