The invention discloses a tea saponin-reduced graphene oxide composite material and a preparation method thereof. The tea saponin-reduced graphene oxide composite material is of a layered porous nanostructure with a smooth surface, which is formed through adsorbing tea saponin to the surface of reduced graphene oxide and bonding the tea saponin to the reduced graphene oxide in a chemical bond manner. The preparation method comprises the following steps: adding sodium borohydride and the tea saponin into a dispersion solution of graphene oxide, and carrying out stirring under a heating condition, so as to produce a precipitate reactant; carrying out aftertreatment on reaction solution from which the precipitate reactant is produced, thereby obtaining the tea saponin-reduced graphene oxide composite material. The tea saponin-reduced graphene oxide composite material has high adsorption capacity to metal cations in wastewater and is low in biotoxicity; the preparation method is simple to operate and low-cost, and is applicable to industrial production.