The invention relates to a preparation method a flame-retardant resin. The preparation method adopts crosslinking chemical reaction, and comprises the following steps: (1) at normal temperature, weighing all the raw materials according to parts by weight for later use; (2) at normal temperature, dissolving maleic anhydride, tetrachloro phthalic anhydride, neopentyl glycol, a curing agent chlorinated styrene and methyl methacrylate, and uniformly mixing by sufficient stirring; (3) curing: curing the uniformly mixed solution obtained in the step (2) to obtain an unsaturated polyester resin, wherein the initial curing temperature is 20-30 DEG C, the initial curing time is 10-30 hours, the postcuring temperature is 25-35 DEG C, and the postcuring time is 4-6 hours; and (4) at normal temperature, mixing and dissolving fibers, copper naphthenate, cobalt isooctoate, calcium carbonate and silicon dioxide in the unsaturated polyester resin prepared in the step (3), uniformly mixing by sufficient stirring, and aging at 25 DEG C for at least 36 hours to obtain the flame-retardant resin.