The foamable phenolic resin was prepared by gradual copolymerization of formaldehyde, paraformaldehyde and phenol using sodium hydroxide (NaOH) as catalyst, by way of adding NaOH, paraformaldehyde in different steps. The environmental protection vesicant, foam stabilizer and mixed acid curing agent were mixed with the foamable phenolic resin to prepare flame-retardant insulation phenolic foam. The influence of the formaldehyde/phenol molar ratio on the activity, toxic residue of foamable phenolic resin was investigated. Besides, the foam cell structure, insulation and flame-retardant properties of the phenolic foam were also studied. The results showed that as the molar ratio of formaldehyde to phenol was 2.0, the free phenol content was 2.3% and hydroxymethyl content was 34.83%, the thermal conductivity was 0.046w/mk, oxygen index was 54.3% and carbon monoxide (CO) peak production was as high as 1.8584 kg/kg, which was suitable to be used as insulation and flame-retardant materials in buildings.