NOVELTY - Straight-flow type biological nitrogen removal sewage treatment comprises: letting sewage of ammonia-nitrogen concentration less than 60 mg/l to an oxygen deficit pool from a first-grade aeration pool located at the initial end with full flow rate and then to a second-grade aeration pool located at the tail end to discharge, where there is no backflow and the concentration of the dissolved oxygen in the first-grade aeration pool is more than 5 mg/l and the removal rate of the biological oxygen demand 5 is less than 50%; and placing a biological mass in the oxygen deficit pool.    USE - The method is useful for straight-flow type biological nitrogen removal sewage treatment.    ADVANTAGE - The method: acquires a dynamic balance of the total amount of aerobic, facultative anaerobic and anaerobic bacteria by combining the application of three-phase biological mass via improving the anaerobic-anoxic-oxic process flow; increases the denitrifying efficiency to more than 90% with stability by reducing the time difference between the nitration of ammonia nitrogen and anti-nitration and that between the anaerobic absorption and aerobic absorption; saves 0.1-0.2 kW/m3 of power consumption and is cost effective, with 0.07-0.12 Yuan/m as the operating cost.    DETAILED DESCRIPTION - Straight-flow type biological nitrogen removal sewage treatment comprises: letting sewage of ammonia-nitrogen concentration less than 60 mg/l to an oxygen deficit pool from a first-grade aeration pool located at the initial end with full flow rate and then to a second-grade aeration pool located at the tail end to discharge, where there is no backflow and the concentration of the dissolved oxygen in the first-grade aeration pool is more than 5 mg/l and the removal rate of the biological oxygen demand 5 is less than 50%; placing a biological mass (50-70% of the effective volume of the oxygen deficit pool) in the oxygen deficit pool, where: the biological mass comprises a shell and an inner core, the shell is a ball which is formed by 10-12 arc-shaped plastic sheets of porosity rate 90-94%, the inner core located at the center of the ball is a hydroformylated fiber ball, and the porosity rate is dispersed and increased from the inner core located at the center of the ball to the outside.