In this study, polyethylene glycol (PEG) was added to phenol-urea-formaldehyde foam to improve its toughness, and the effects of PEG, with different molecular weights and dosages, on the mechanical property, microstructure, thermal stability, and flame resistance of phenol-urea-formaldehyde foam were studied. The addition of PEG significantly increased the toughness and impact strength and decreased the pulverization rate of the foam. The compression strength of the foam first increased and then decreased with increasing amounts of PEG. When 2 wt% PEGs were added, the compression strength of foams was the highest. The addition of PEG significantly influenced the microstructure of phenol-urea-formaldehyde foams, in which the cell diameter decreased and wall thickness increased with increasing amount and molecular weight of PEG. The addition of PEG also slightly decreased the thermal stability of phenol-urea-formaldehyde foams, and increased the heat release rate, total heat release, and total smoke release of the foams.