The invention discloses a preparation method of a hindered phenol antioxidant 1,3,5-tri(4-tertiary butyl-3-hydroxyl-2,6-dimethyl phenyl)-1,3,5-triazine-2,4,6-(1H-3H-5H)-triketone, 1,3,5-triazine-2,4,6-(1H-3H-5H)-triketone and 2,6-dimethyl-4-tertiary butyl-3-hydroxyl benzyl methyl ether are taken as the raw materials, and the 1,3,5-tri(4-tertiary butyl-3-hydroxyl-2,6-dimethyl phenyl)-1,3,5-triazine-2,4,6-(1H-3H-5H)-triketone is prepared through the reaction under the condition that a catalyst and a solvent exist. The molecular weight of the hindered phenol antioxidant is large, the migration ratio is relatively low, the heat stability is relatively high, and the compatibility with a matrix, particularly a resin material, is relatively good. The 1,3,5-triazine-2,4,6-(1H-3H-5H)-triketone andthe 2,6-dimethyl-4-tertiary butyl-3-hydroxyl benzyl methyl ether are taken as the raw materials to synthesize the 1,3,5-tri(4-tertiary butyl-3-hydroxyl-2,6-dimethyl phenyl)-1,3,5-triazine-2,4,6-(1H-3H-5H)-triketone, the separation and purification processes are simple, the reaction time is short, the product yield is 90% or more, the product purity is high, the energy consumption is low, the environment pollution is low, the cost is low, and the preparation method is a relatively ideal process for implementing the industrial production.