Disclosed is a method for preparing a composite antioxidant for better improving the yellowing of a polymer caused by processing and long-term aging. The composite antioxidant is prepared by using a main antioxidant, an auxiliary antioxidant, a dispersant, an organic pigment, and an inorganic pigment as main raw materials, wherein, first, the main antioxidant and the auxiliary antioxidant are mixed and melted to form a molten liquid, then, the dispersant is added and mixed until uniform, then same is ultrasonically treated under an ultrasonic wave, an organic pigment powder and an inorganic pigment powder are added and dispersed uniformly, and finally, extrusion granulation is performed via a twin screw extruder to obtain composite antioxidant particles. By compositing the molten liquid of the main and auxiliary antioxidants with the pigments, and combining the function of the dispersant and the ultrasonic technology, the pigment powders are more uniformly dispersed into the molten liquid of the antioxidants, such that the pigment content in a single composite antioxidant particle is uniform; furthermore, by compounding the composite antioxidant with a polymer, the color complementary effect of the pigment on the coloring of the polymer is improved, and yellowing of the polymer caused by processing and long-term aging is effectively improved.