A method for preparing a phenolic oligomer antioxidant with a very small residual content of butylated hydroxytoluene (BHT) is provided.Specifically, the method for preparing the phenolic oligomer antioxidant relates to a method which is capable of a BHT-free phenolic oligomer antioxidant or a phenolic oligomer antioxidant containing a trace amount of residual BHT by removing the precursor of BHT as much as possible by performing the concentration under the reduced pressure while injecting an inert gas to an intermediate product and/or removing BHT by performing the concentration under the reduced pressure while injecting an inert gas to the final product.