Since the ammonia in the effluent of the traditional water purification process could not meet the supply demand, the advanced treatment of a high concentration of 4-N micro-polluted source water by biological activated carbon filter (BACF) was tested. The filter was operated in the downflow manner and the results showed that the removing rate of NH4-N was related to the influent concentration of NH4-N. Its removing rate could be higher than 95% when influent concentration was under 1.0 mg/L. It could also decrease with the increasing influent concentration when the 4-N concentration was in the range from 1.5 to 4.9 mg/L and the dissolved oxygen (DO) in the influent was under 10 mg/L, and the minimum removing rate could be 30%. The key factor of restricting nitrification in BACF was the influent DO. When the influent NH4-N concentration was high, the DO in water was almost depleted entirely by the nitrifying and hetetrophic bacteria in the depth of 0.4 m filter and the filter layer was divided into aerobic and anoxic zones. The nitrification and degradation of organic matters existed in the aerobic zone, while the denitrification occurred in the anoxic zone. Due to the limited carbon source, the denitrification could not be carried out properly, which led to the accumulation of the denitrification intermediates such as NO2. In addition to the denitrification bacteria, the nitrification and the heterotrophic bacteria existed in the anoxic zone.