This study focused on domestic wastewater in a campus and investigated chemical oxygen demand (COD) removal efficiency and removal mechanism in a wastewater treatment system using three-step series constructed rapid infiltration (CRI). The experimental results indicated that the three-step series CRI system led to COD removal of 79.65%, a value 6% greater than that in a system using conventional CRI. Moreover, the effluent water quality of the three-step series CRI system met the first level criteria A as specified in Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002). Total organic matter and particulate organic matter in the wastewater were removed in the first compartment of the system by 54% and 83%, respectively. The quantity of aerobic ammonifying bacteria decreased from top to bottom along the compartments in the system, and was obviously higher than in the conventional CRI system. The dehydrogenase activity showed an M-shaped variation pattern in which two peak activity values were observed. The COD removal efficiency showed a very significant correlation with the aerobic bacteria quantity (correlation coefficient r = 0.97), and showed a relatively significant correlation with the dehydrogenase activity (r = 0.64). The biofilm mass and the active biofilm mass showed M-shaped variation patterns, and were obviously higher than in the conventional CRI system. The COD removal efficiency showed a relatively significant correlation with the weight of the active biofilm mass (r = 0.73).