The guidelines of Activated Sludge Model (ASM) can make the use of modeling more straightforward and systematical. Therefore, this paper aims to develop a guideline for wastewater COD characterization. Four physical-chemical methods were compared experimentally for wastewater characterization in this paper and flocculation + 0.45 μm membrane filtration combining respirometry was selected to determine soluble inert COD. Simultaneous determination of readily and slowly biodegradable COD through respirometric measurement of raw wastewater was proved feasible. In addition, two methods were evaluated to determine heterotrophic biomass. In light of these investigations, an integrated system for characterizing wastewater COD fractions defined in ASM1 was proposed and then applied to characterize influent COD of a treatment plant. By following this guideline, a detailed and practical COD characterization can be obtained. Moreover, this study could also provide an example for the development of more widely accepted guidelines.