The mechanism and the modeling process of the activated sludge system simulation software GPS-X are described, and combined with the results of characterization of COD components, an instance of the software’s application in a sewage treatment plants is analyzed. The simulation results present the COD with a maximum error of 5.48%, a minimum error of 0.83%, and an average relative error of 2.85%, TN with a maximum error of 7.23%, a minimum error of 0.15%, and an average relative error of 3.62%, and NH+4-N with a maximum error of 9.17%, a minimum error of 1.05%, and an average relative error of 4.98%. The simulation results match the measured results and reflect the actual situation of sewage treatment plants preferably.