The invention relates to a preparation method of a bis(2,4-dicumylphenyl)pentaerythritol diphosphite antioxidant. The preparation method comprises the steps of dropwise adding phosphorus trichloride at a nitrogen environment in an existing system of pentaerythritol, a catalyst and a solvent so as to obtain dichlone pentaerythritol diphosphite reaction liquid, adding 2,4-dicumylphenol, heating to 130-140 DEG C in the presence of nitrogen to react, carrying out reduced pressure backflow reaction to obtain a reactant, adding organic amine to neutralize and regulate the reactant to be alkaline, continuing to carry out esterification reaction until reaction mediums are still alkaline, cooling, and filtering, so as to obtain bis(2,4-dicumylphenyl)pentaerythritol diphosphite. By controlling the reaction process, high-selectivity and high-yield bis(2,4-dicumylphenyl)pentaerythritol diphosphite with a spiral structure is obtained, the yield of the product is higher than 92%, the recovery rates of generated side effects and an unreacted raw material, namely 2,4-dicumylphenol are higher than 90%, the consumption of the raw materials is reduced, and the simple and feasible operability is realized.