The present invention relates to a method of preparing an ABS-based resin composition and a method of manufacturing an ABS-based injection-molded article including the same. More particularly, the present invention provides a method of preparing an ABS-based resin composition using a multimeric acid of unsaturated fatty acid or a metal salt thereof, as an emulsifier, in a diene-based rubber latex polymerization step and an ABS-based graft polymerization step. In accordance with the present invention, when an ABS-based resin composition is prepared using a multimeric acid of unsaturated fatty acid or a metal salt thereof as an emulsifier, stability of polymerization is secured and surface characteristics, such as surface gloss or sharpness, of an ABS-based injection-molded article are improved.