The invention discloses a foam board which comprises the following components in parts by weight: 60-100 parts of polylactic acid, 30-50 parts of polyethylene, 6-17 parts of polyethylene glycol ether,2-8 parts of polyester polyol, 5-16 parts of polyester resin, 6-18 parts of starch, 1-5 parts of a foam stabilizer, 2-6 parts of a seaweed extract, 1-4 parts of silicone oil, 3-5 parts of high bauxite, 8-15 parts of ethylene-vinyl acetate, 0.5-3 parts of expanded graphite, 3-7 parts of bamboo fiber, 1-4 parts of magnesium carbonate and 0.2-0.5 part of a stabilizer. The preparation method comprises the steps of mixing, ultrasonic treatment, pre-foaming, extrusion and the like. The foam board is low in degradation rate and long in service life, generated degradation products are almost harmlessto human bodies and the environment, and the foam board has good structural strength and fireproof performance.