The invention discloses a flame-retardant block copolymer containing nitrogen and phosphorus and a preparation method thereof and relates to a block copolymer. The flame-retardant block copolymer containing nitrogen and phosphorus is PMOEPm-b-PDMAEMAn. The preparation method comprises the steps of dissolving 2-hydroxyethyl methacrylate phosphate, cumyl dithiobenzoate and azobisisobutyronitrile in a solvent, carrying out freeze thawing and degassing until unfreezing is realized and no bubbles come out, carrying out polymerization reaction under the protection of inert gas, quenching with liquid nitrogen to stop the reaction, precipitating and drying to obtain a product which is a PMOEPm macromolecular chain transfer agent; dissolving the PMOEPm macromolecular chain transfer agent, dimethylaminoethyl methacrylate and azobisisobutyronitrile in the solvent, carrying out freeze thawing and degassing, carrying out polymerization reaction under the protection of inert gas, quenching with liquid nitrogen to stop the reaction and repeatedly precipitating with ethanol used as a precipitant to obtain the target product, namely, flame-retardant block copolymer containing nitrogen and phosphorus (PMOEPm-b-PDMAEMAn). The flame-retardant block copolymer containing nitrogen and phosphorus can be used in preparation of halogen-free flame retardant and self-assemble micelles.