The properties of foam concrete composites filled with different particle size expanded polystyrene (EPS) were studied. The results show that compared with large particles of EPS, the addition of small particles of EPS results in a greater reduction in the apparent density and water absorption of foam concrete, a greater increase in the porosity of pores, a smaller reduction in the flexural strength and compressive strength, and a greater reduction in the thermal conductivity. The drying shrinkage of foam concrete is delayed. When the mass fraction of small particles EPS is 3%, the apparent density is 463 kg/m³, the porosity is 63%, which is 43.18% more than the blank group. The flexural strength and compressive strength are up to 2.43 MPa and 7.1 MPa, and the thermal conductivity is 0.108 1 W/(m·K), indicating that the proper amount of small particles EPS can effectively improve the stability and mechanical properties of foam concrete.