The invention discloses cardanol modified phenolic aldehyde foam and a preparation method of the cardanol modified phenolic aldehyde foam. The cardanol modified phenolic aldehyde foam is prepared by the following materials in parts by weight: 100 parts of a cardanol modified phenolic aldehyde resin, 4-7 parts of a surfactant, 6-10 parts of a foaming agent and 15-20 parts of a curing agent, wherein the cardanol modified phenolic aldehyde resin is a phenolic aldehyde resin obtained by performing a first step of reaction on cardanol and liquid formaldehyde under a basic catalyst condition, then, adding phenol and paraformaldehyde, and performing a second step of reaction under the basic catalyst condition, and at last, adding a hydroxymethylurea resin to react. According to the preparation method, on the basis of not affecting the heat conductivity coefficient and high fire resistance of the phenolic aldehyde foam, the density, water absorption and free formaldehyde of phenolic aldehyde foam plastic are reduced by the cardanol, the tenacity of phenolic foam is improved, and meanwhile, renewable plant resources are adequately used. The cardanol modified phenolic aldehyde foam can be used as a building heat-insulation, heat-preservation and fire-proof material, and is a product with good market prospect.