The invention relates to polyurethane foam insulation materials, in particular to a preparation method of rigid flame-retardant polyurethane foams. The preparation method is characterized in that a flame retardant is added to premixed polyether polyol in a preparation process and mixed uniformly; then polyisocyanate is added and is poured into a mold instantly for foaming when being stirred until foams are white, after the foams are swelled to be molded, the foams are taken out and subjected to room-temperature curing, and the rigid flame-retardant polyurethane foams are prepared. According to the preparation method, aluminum hydroxide powder is modified successfully with a wet process modification method, so that the aluminum hydroxide powder and a polyurethane matrix have good compatibility; the rigid flame-retardant polyurethane foams have low probability of combustion, good flame retardance and low flame propagation velocity and are free of smoke generation; meanwhile, a one-step foaming method is adopted, the additive flame retardant is adopted, the technical process is simple, the cost is low, technical scale-up is facilitated, and the method is suitable for industrial production.