The invention relates to a preparation method of a dry powder extinguishing agent containing allophanate, and belongs to the technical field of extinguishing materials. The dry powder extinguishing agent is obtained by prefabricating extinguishing components through a mechanochemical method, synthesizing the extinguishing components through a hydrothermal / solvothermal method and then performing surface modification through a modifier. The fire extinguishing components of the fire extinguishing agent comprise potassium allophanate and / or sodium allophanate, and at least two of potassium bicarbonate, sodium bicarbonate, zinc carbonate, zinc borate, zirconium phosphate, manganese hypophosphite, ammonium phosphomolybdate, ammonium molybdate, hexadecyl trimethyl ammonium bromide, urea, ferrocene, molybdenum disulfide, ferric chloride, aluminum silicate and magnesium hydroxide. The prepared dry powder extinguishing agent can absorb heat in a fire scene and release non-combustible gas, a large number of active chemical particles are generated to capture flame free radicals, and the extremely outstanding extinguishing effect is achieved. In addition, through surface modification of the modifier, the dry powder extinguishing agent further has good hydrophobicity and flowability, and storage, transportation and application of the extinguishing agent are better facilitated.