The invention relates to an environmental-friendly flame retardant for epoxy resin and a preparation method thereof. The method comprises the following steps of: compounding tea saponin, ammonium polyphosphate and pentaerythritol in a certain proportion into a novel flame retardant; fully grinding the flame retardant into superfine powder and adding the powder into the epoxy resin in a certain proportion; uniformly mixing the powder and the epoxy resin and adding a coupling agent and a polyamide curing agent into the mixture; and stirring the mixture uniformly and pouring the mixture into a mold frame to obtain a flame retardant epoxy resin product. In the flame retardant for the epoxy resin prepared by the method, a good cooperative flame retarding effect among an acid source, a carbon source and a gas source (tea saponin) is achieved. Compared with a conventional expansible flame retardant, the flame retardant has the advantages of lower cost, good flame retarding and smoke restraining effects, high mechanical property, safety, environmental friendliness and the like.