The invention discloses a 2,4,6-triphosphate diethyl hydroxymethylphenoxy-1,3,5-triazine flame retardant and a preparation method thereof, belonging to the technical field of flame retardancy. Add cyanuric chloride, p-hydroxybenzaldehyde, and a catalyst to an organic solvent, heat to reflux for reaction, and then undergo a post-processing step to obtain a white solid powder 2,4,6-trialdehyde-phenoxy-1,3,5 ‑triazine; then 2,4,6‑3 formylphenoxy‑1,3,5‑triazine, diethyl phosphite, and triethylamine are heated and refluxed in an organic solvent, and the reaction product is decompressed After distilling off the organic solvent and triethylamine, a 2,4,6-diethyl triphosphate hydroxymethylphenoxy-1,3,5-triazine flame retardant is obtained. The preparation process of the 2,4,6-triphosphate diethyl hydroxymethylphenoxy-1,3,5-triazine involved in the present invention is simple and easy to operate, has high yield, strong practicability, environmental protection and pollution-free, and can large-scale industrial production.