Nitrifying activated sludge (NAS) was enriched in a membrane bioreactor (MBR) with pre-treated municipal wastewater and additional ammonium sulfate as the culture medium. The influences of temperature, dissolved oxygen (DO), ammonia nitrogen volumetric load, free ammonia (FA), and free nitrite (FNA) on the enrichment of NAS were investigated, the cost of the process was evaluated, and then NAS's application in enhancing a wastewater biological treatment system against ammonia shocking loads was attempted. The results showed that after 182 days of cultivation in an MBR, NAS had a nitrification activity of 98.41 mg·(L·h), which was 30-times higher than that of the seeding sludge. The yield of NAS was 14.96 mg·(L·d), costing 3.52 Yuan for 1 kg. Temperature was found to be a key factor affecting the sludge nitrification activity. The sludge nitrification activity was decreased to 1/3 of the maximum value at temperatures below 15.0℃, while lowering the ammonium volumetric load retarded the decrease in the sludge nitrification activity to some extent. In addition, dissolved oxygen deficiency resulted in nitrite accumulation, and thereby slowed down the NAS enrichment rate. The enriched NAS was then applied to a wastewater biological treatment pilot equipment, which had just been exposed to an ammonium shocking load. The removal rate of ammonia nitrogen in the biological system increased from 29.4% to 88.4% after 2.0% of NAS was inoculated. The enhanced biological system retained ammonia removal rates of as high as 99.0%, even as the temperature dropped to 13.3℃±1.6℃ afterwards. The above pilot-experiment results suggested that enriched nitrifying sludge is suitable for quickly increasing the start-up or recovery rates of the nitrifying function in a biological system.