The invention provides a preparation method for a flame-retardant softener applied to styrene butadiene rubber. The preparation method comprises the following steps: dissolving styrene butadiene rubber in a solvent to prepare a solution, then adding an olefin metathesis catalyst and a rubber fire retardant and subjecting a reacted rubber solution to solvent removal so as to obtain a flame-retardant softener product. The flame-retardant softener allows the Mooney viscosity of basic rubber to decrease by 0.2 to 2.0 units/part and the oxygen index of basic rubber to increase by 0.1 to 1.0%/part when added into the styrene butadiene rubber on an open mill.