The invention provides a biomass graphene-modified fire retardant and master batch and a preparation method thereof. The biomass graphene-modified fire retardant is mainly prepared from biomass graphene, a fire retardant and a coupling agent. The fire retardant is used as a disperse medium for the biomass graphene. By simple physical mixing, bond joint of the biomass graphene with the fire retardant and uniform dispersion of the biomass graphene are realized, thus finally playing a synergistic flame retardant effect in a polymer matrix. By the adoption of the Biomass graphene-modified fire retardant for preparation of high-molecular polymer master batch, the dispersion problem of biomass graphene is solved, and it is possible to effectively raise fire resistance under the condition of low addition amount of the fire retardant. For practical application fields of different fire-resistant high-molecular polymer materials, anti-dripping, enhancing, ultraviolet-resistant and coloring functions can be obtained simultaneously by the adoption of the master batch product.