The invention discloses polyurethane foam which is prepared from the following raw materials in parts by weight: 90-100 parts of pentadecyl alkylphenol polyether polyol, 100-110 parts of polymethylene polyphenyl isocyanate, 25-30 parts of expansible graphite, 1-2 parts of a foam stabilizer and 20-25 parts of a foaming agent, wherein pentadecyl alkylphenol polyether polyol is prepared from the following raw materials in parts by weight: 20-25 parts of 3-pentadecylphenol, 15-20 parts of diethanol amine, 3-4 parts of formaldehyde, 0.5-1 part of triethylamine and 3-4 parts of epoxypropane. Pentadecyl alkylphenol polyether polyol used in the formula of the polyurethane foam is extracted from natural cashew nut shells, is a biomass raw material, is an ideal substitute of petrochemical material phenol and has the advantages of no toxicity and biodegradability. Expansible graphite used as a fire retardant is added into the formula disclosed by the invention, so that the flammable risk of the polyurethane foam is effectively reduced.