The invention discloses a phosphorus-nitrogen synergistic flame-retardant hard polyurethane foam plastic (cross-linking network structure). The phosphorus-nitrogen synergistic flame-retardant hard polyurethane foam plastic comprises the following raw materials in parts by weight: 95-110 parts of polyether polyol and reaction type phosphorus-nitrogen flame retardant, 2-3 parts of an amines catalyst, 1-2 parts of a tertiary amines catalyst, 1-2 parts of a foam stabilizer, 1-2 parts of a foaming agent and 140-150 parts of isocyanato. The reaction type phosphorus-nitrogen flame retardant is glycol of a phosphorus-nitrogen-containing flame-retardant element, functions of the reaction type phosphorus-nitrogen flame retardant are similar to those of the polyether polyol, the reaction type phosphorus-nitrogen flame retardant can chemically react with the isocyanato to generate a carbamate group, phosphorus and nitrogen which are flame-retardant elements are introduced into a cross-linking polyurethane network in a covalent chain mode, shortcomings of the traditional addition type flame retardant in a physical blending mode are overcome, and the hard polyurethane foam plastic which is compatible in a forced manner and is synergistically flame-retardant is formed.