This paper focused on the effluent chroma which is a long-term puzzle about the aerobic treatment for pulping midcourse wastewater. The organic in the influent and effluent was tested by GC-MS. The analysis indicates that the macromolecular lignin-type substances are partly degraded using aerobic technique. Some micromolecular substances are produced accordingly, which leads to the auxochrome groups and chromophore groups increasing. There are two organic coloration compositions in influent but three in effluent, which account for 22.98% of the organic substances in influent but 33.12% of the organic substances in effluent.