The invention discloses halogen-free and flame-retardant hard polyurethane foam plastic and a preparation method thereof, relating to foam plastic. Raw materials include polyether polyol 4110, polymethylene polyphenyl isocyanate, a tertiary amine catalyst, dibutyltin dilaurate, a silicon-carbon bond foam stabilizer, 2,4,6-tri(dimethylaminomethyl)phenol, a foaming agent, octa-(3-hydroxyl-3-methyl butyl dimethylsiloxy)silsesquioxane, dimethyl methylphosphonate and micron-sized alpha-alumina trihydrate. The method comprises the following steps of: mixing the polyether polyol 4110, tertiary aminecatalyst, dibutyltin dilaurate, silicon-carbon bond foam stabilizer, 2,4,6-tri(dimethylaminomethyl)phenol, foaming agent, octa-(3-hydroxyl-3-methyl butyl dimethylsiloxy)silsesquioxane and micron-sized alpha-alumina trihydrate to obtain A; adding the dimethyl methylphosphonate into A to obtain B; and adding the polymethylene polyphenyl isocyanate into B, pouring into a mold for foaming, curing anddemolding to obtain the halogen-free and flame-retardant hard polyurethane foam plastic.