Achieving partial nitrification (PN) in low-strength wastewater is still a worldwide issue. In this study, conversion from nitrification to PN was successfully achieved with a self-sustained pH as low as 5.3 and a nitrite accumulation ratio (NAR) of 93.4 % by enriching novel acidic-tolerant functional flora. A strong positive correlation was observed between decreasing pH and NAR (R2 = 0.97). Even with 73.1 % reduction in influent ammonia (from 86.7 to 23.3 mg/L), high dissolved oxygen loading (4.5–5.3 mg/L), and extreme over-aeration, PN efficacy remained robust with the NAR consistently maintained at 90.6 ± 5.3 %. Over 180 days of continuous monitoring, novel ammonia-oxidizing bacteria (AOB) Candidatus Nitrosoglobus became the dominant nitrifying guild (1.65 %) with a specific growth rate of 0.091/day; however, traditional nitrifying bacteria exhibited impaired functionality and bacterial diversity was dramatically reduced. Amo abundance increased by 147 %, while Nxr in nitrifying bacteria dropped below detection limit, preventing complete nitrification. Upregulated electron transport chain genes enhanced electron flow, managing excess oxygen and effectively limiting harmful oxygen radical production, resulting in a desirable nitritation rate of 0.3 kg/m3·d with influent NH4+-N as low as 23.3 mg/L. These regulatory mechanisms boosted NH4+ oxidation efficiency, successfully unlocking scientific barriers associated with PN in ultralow-strength wastewater and laying the groundwork for future PN/A implementation.