A biological nitrogen removal system for high ammonia-nitrogen coal gasification waste water is composed of a first-stage biochemical tank and a first-stage secondary sedimentation tank and comprises four reaction zones and a degassing zone.The system is characterized in that the four reaction zones include the two-stage anoxic zones and the two-stage aerobic zones which are the pre-denitrification zone, the aerobic zone, the post-denitrification zone, the rear aeration zone and the degassing zone; the pre-denitrification zone receives a reflux mixing solution of the aerobic zone and returned sludge of the secondary sedimentation tank on the downstream part; the post-denitrification zone is provided with an acid and carbon source dosing system; the biological nitrogen removal system is compact in structure, small in occupied area and high in denitrification rate for coal gasification waste water with high toxicity and low biological fitness, and the two-stage anoxic and aerobic zones are flexible in design, stable in running and high in impact load resistance.