The invention provides a synthetic method for tricyanic acid spiro-phosphate, which relates to a synthetic method for an intumescent flame retardant. The invention solves the problem of dramatic decline in mechanical properties, which results from the uneven component distribution of the existing intumescent flame retardant. The synthetic method comprises the steps that: 1. clear solution A is prepared; 2. solution B is prepared; 3. anhydrous polar solvent in solution B is distilled, is washed and is dried in vacuum to obtain the tricyanic acid spiro-phosphate. The invention synthesizes the three-in-one intumescent fire retardant which is a chemical compound with a new structure, and the intumescent fire retardant has preferable intumescent ignition retardance and no decrease in mechanicalproperties. The invention has simple preparation technology, easy synthesis, low cost and large-scale industrial production.