Display omitted •Rapid start-up of A/O/A-SNDPR was achieved within 15 days.•With significant in situ sludge reduction, Yobs was as low as 0.017 g MLSS/g COD.•Under the condition of long SRT without sludge discharge, the removal rate of TP reached 96.37%.•Candidatus_Competibacter was enriched up to 29.06 % in the system. An anaerobic/aerobic/anoxic mode simultaneous nitrification, denitrification, and phosphorus removal system was visited for enhanced low-strength wastewater treatment and dramatic in situ sludge reduction. Results showed that rapid start-up was achieved with conventional activated sludge after 15 days, with effluent ammonia nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand being 0.25, 7.89, 0.12, 24.37 mg/L, respectively. A two-stage biomass growth rate was observed with the sludge yield of 0.285 (day 1–50) and 0.017 g MLSS/g COD (day 51–110) without sludge discharge. Dynamics of bacterial community has been identified with outstanding accumulation of Candidatus_Competibacter up to 29.06 %, which contributed to both simultaneous nutrients removal and sludge reduction. Further analysis via PICRUSt2 revealed the main pathway of nitrogen metabolism, while proposed mechanism for phosphorus removal with no sludge discharge was analyzed from the intracellular and extracellular perspectives. Overall, this study provided guidance and reference for the development and application of A/O/A-SNDPR technology.