The composition analysis of the acrylonitrile butadiene styrene (ABS) copolymer was performed using a method combining elemental analysis and 1H nuclear magnetic resonance (NMR) spectroscopy. The physicochemical properties of ABS are determined by the relative contents of the constituent monomers. However, because ABS undergoes a complicated microstructural change during the polymerization process, it is difficult to accurately analyze the relative contents using only a typical method. In this study, a combination of nitrogen elemental analysis and 1H NMR spectroscopy was proposed as an analytical method that can consider the effects of changes in the microstructures. This approach confirmed whether the ABS samples were close to meeting the specifications. In addition, the composition analyses of polylactic acid/ABS blends, polycarbonate/ABS blends, and a polyamide 6/ABS mixture sample indicate that the obtained results were consistent with the expected values, thereby demonstrating that this combined analytical approach can be applied to various types of ABS copolymer systems.