The invention discloses a preparation method of intumescent flame-retardant polyurethane in the technical field of high polymer materials. The preparation method of the intumescent flame-retardant polyurethane is characterized by comprising the following steps: preparing materials, polyether triol, polyoxyethylene glycol, a foaming agent urea, a foaming agent carbonic acid organic alcohol amine salt, potassium ethylhexanoate, dibutyl dilaurate, a foam stabilizer, a copolymerization monomer flame retardant, nano aluminum hydroxide, ammonium octamolybdate and a plurality of parts of isocyanate,and stirring and dispersing: adding polyether triol, polyoxyethylene glycol, a foaming agent urea, a foaming agent carbonic acid organic alcohol amine salt, potassium 2-ethylhexanoate, dibutyl dilaurate, a foam stabilizer, a copolymerization monomer flame retardant, nano aluminum hydroxide and ammonium octamolybdate into a dispersing machine. The invention effectively solves the problems of dispersity and compatibility of the organic flame retardant in the polyurethane resin, enhances the flame-retardant efficiency of the flame retardant, and enhances the mechanical properties of the polyurethane foam plastic.