A biological nitrogen removal method based on multistage feeding and anoxic/aerobic chambers for adopting a constant flow operation mode, enabling wastewater subjected to primary treatment to enter a first-stage and a second-stage A/O reaction units from two position points; lifting and refluxing an effluent to an influent end of the first-stage units; lifting and refluxing an effluent to an influent end of the second-stage units; and clarifying, separating and discharging an effluent from an outlet end of a third-stage A/O reaction unit. The wastewater treatment mode combines a process based on two-stage wastewater feeding and three-stage anoxic/aerobic chambers with biofilms in different reaction zones, =optimizing and controlling a reflux ratio of each segment according to an amount of organic matters available for denitrification in influent, reducing the impact of hydraulic loading at the cross section in the reactors while removing organic matters and nitrogen-containing pollutants at a low temperature.