The invention discloses a method for synthesizing dithio dineopentyl phosphate fire retardant. The synthesis process comprises: (a) filling a solvent A in a stirred vessel, adding dineopentyl alcohol, fully stirring, adding phosphorus thiochloride, and fully dissolving at a constant temperature of 40 to 55 DEG C; (b) dripping solvent B, heating to 50 to 65 DEG C, and keeping the temperature for 1.5 to 2.5 hours; (c) adding a solvent C and water, heating to 60 to 75 DEG C and keeping the temperature for 8 to 12 hours; and (d) filtering and grinding to obtain dithio dineopentyl phosphate fire retardant suspension, which is the product of the invention. The solvent A is cyclohexane, the solvent B is triethylamine or the mixed solution of cyclohexane and triethylamine, and the solvent C is triethylamine. In the invention, the process is simple and short in process, the raw materials are widely available and cheap, the yield is high and the industrial prospect is promising; the dithio dineopentyl phosphate fire retardant contains the phosphorus and sulfur high-efficiency fire-retarding elements at the same time, so the fire retarding performance of cellulose is improved considerably under the synergistic effect of the phosphorus and sulfur; and the dithio dineopentyl phosphate fire retardant is high-efficiency and durable fire retardant.