The invention relates to a biological nitrogen removal technology implemented under the low temperature and low carbon source conditions. An up-flow anoxic/anaerobic reactor (UAAR), a nitration tank and a sedimentation tank are involved in the technology. The UAAR integrates the functions of primary sedimentation, anaerobic treatment, anoxic treatment and sludge thickening. A bottom sludge hopper, with a better water flow state and a larger effective depth, in the reactor provides conditions for sludge to be subjected to anaerobic acidification and generate VFAs in the sludge hopper, a carbon source easy to use biologically is supplemented to the system, and therefore the system can achieve nitrogen removal under the low temperature and low carbon source conditions. Compared with the prior art, only three structures are involved in the technology, the technology is simple, the tedious arrangement of multiple tanks is omitted, the occupied area is reduced, and the construction cost is lowered; an up-flow push mode is adopted for the main structure, namely the novel UAAR, high sludge concentration can be kept, high impact loads can be borne, and through up-flow mixing, stirring energy needed by a traditional technology is greatly lowered; a good nitrogen removal effect can be achieved without adding carbon sources, and the COD of effluent at low temperature, the ammonia nitrogen concentration and the total nitrogen concentration can reach the level A standard; the sludge concentration is high, the SRT is long, and the amount of excess sludge is greatly reduced.