The invention discloses a preparation method of a phosphorus-nitrogen-carbon-containing aggregate and intumescent flame retardant of a nano-sheet structure. The preparation method is characterized in that phytic acid, melamine and a solvent are prepared, the substance amount ratio of the phytic acid to the melamine is 1: 3 to 1: 15, and 20-50 ml of the solvent is added into every 0.01 mol of the melamine; the melamine is added into a reaction container provided with a thermometer and a stirrer, the solvent is added with stirring, the phytic acid is added at the temperature of 20-40 DEG C, and a reaction is conducted with stirring for 20-50 minutes, so that reaction liquid is obtained; and the reaction liquid is poured into a reaction still to react for 24-56 h at the temperature of 70-120 DEG C, cooling and suction filtration are conducted, solid is washed with ethyl alcohol and then washed with water till the pH is 7, and the solid is dried, so that the flame retardant is obtained. The preparation method is simple and environmentally friendly, and the synthesized phosphorus-nitrogen-carbon-containing aggregate and intumescent flame retardant of the nano-sheet structure has good compatibility with a polymer matrix and has good flame retardance.