The study explored a novel method for achieving stable partial nitrification (PN) in mainstream municipal wastewater A-stage effluent treatment by integrating an anaerobic side-stream reactor with a mainstream B-stage nitritation reactor with biofilm and granulation. With influent COD/N and COD/P ratios of 2.60 and 27.1, respectively, the system showed stable nitritation with a nitrite accumulating ratio (NAR) of 95.1% via partial nitrification. Both kinetic assessment and 16S ribosomal RNA (16S rRNA) sequencing and functional gene marker quantification confirmed successful nitrite-oxidizing bacteria (NOB) out-selection (<0.05% relative abundance). None of the commonly employed approaches for NOB out-selection, including low dissolved oxygen (DO), high pH, and high concentrations of free nitrous acid (FNA) and/or free ammonia (FA), occurred in our study. During the stable period, approximately 90% of the total biomass was in the biofilm in the mainstream sequencing batch reactor (SBR), with the remaining 10% of the biomass in suspension as granules. The selective wasting strategy of wasting the suspended biomass from the mainstream SBR contributed to the formation of granulation. The substrates and oxygen gradient along the depth of the biofilm's layered structure, alongside the unique conditions in the anaerobic side-stream reactor, were suggested to play roles in NOB out-selection. This study demonstrated a new strategy that integrates an anaerobic side-stream reactor with mainstream B-stage nitritation for short-cut nitrogen removal with granulation.