The invention relates to a preparation method of a graphene/sodium alginate composite material. The preparation method includes the steps of: 1) ultrasonically dispersing graphene oxide in water to obtain a dispersion liquid, and adding a stabilizing agent to the dispersion liquid and continuously performing ultrasonic treatment; 2) adding the graphene oxide/stabilizing agent dispersion liquid to a sodium alginate water solution, stirring the mixture at high speed and then at low speed at room temperature, and adding a reducing agent to performing an oil-bath reaction; 3) allowing the mixture to stand, drying and evaporating the solvent to obtain graphene/sodium alginate composite membrane; and 4) performing crosslinking in a 4% calcium chloride water solution for 30 min, washing the membrane in clear water, and drying the membrane to produce the graphene/sodium alginate composite material. The graphene/sodium alginate composite membrane is improved in mechanical performance, heat resistance, flame-retarding property, and heat conductivity and electric conductivity. The preparation method employs green raw material, is free of environment pollution, and is convenient to use in industrial production.