The invention discloses flame retardant polyol composition. The composition comprises P-N containing polyol and polyether polyol, wherein the P-N containing polyol is prepared from P-containing smallmolecules and N-containing small molecules by reaction with polyol and polybasic acid; the N-containing small molecules are trimethylol melamine or tris(2-hydroxyethyl)isocyanurate. The invention further discloses a preparation method of P-N containing polyol of the flame retardant polyol composition and an application of the flame retardant polyol composition to polyurethane rigid foam plastic. Flame retardant elements P and N are both introduced into the molecular chain of polyurethane by chemical reaction, and the composition has better flame retardant effect by synergistic effect of N-P, and has lower oxygen index, and prepared polyurethane has lower thermal conductivity and better comprehensive performance.