The invention provides a flame retardant for high density polyethylene. The flame retardant is prepared from the following raw materials in parts by mass: 6-24 parts of decabromodiphenyl ethane, 2-8 parts of Antimony trioxide, 2-8 parts of modified calcium base bentonite (for 100 parts by mass of high density polyethylene base material). According to the flame retardant for high density polyethylene, the used modified bentonite is abundant in resources and low in price; alkaline calcium-based bentonite and halogen-antimony flame retardant are used for synergistic flame retarding, so that the using amount of halogen-antimony flame retardant can be reduced, and the flame retarding cost of high density polyethylene can be lowered; for the high density polyethylene with purer high density polyethylene flame retarding materials, the oxygen index can be increased by more than 10%, and the thermal weight loss rate can be reduced by more than 4.94%. The flame retardant combines the advantages of halogen flame retardants and inorganic flame retardants, so that the melt dripping does not occur in the process of combustion, the char yield is high, the carbon layer is stable, and toxic gases are not generated. The alkaline calcium-based bentonite is added to improve the processing properties of materials and increase the elastic modulus of materials.