Water　quantity　adjustment　equipment　was　installed　to　balance　the　influent　quantity　of　different　paralleling　treatment　units　in　a　WWTP　of　400000　m3/d　to　tackle　the　problem　of　poor　treatment　effect　resulting　from　the　uneven　distribution　of　influent　wastewater　to　the　treatment　units　of　the　large-scale　WWTP.　Water　level　change　and　concentrations　of　NH4+-N,　NO3--N,　NO2--N　along　the　bio-reactor　were　compared　before　and　after　installing　the　water　quantity　adjustment　equipment.　Result　shows　that　the　influent　water　quantity　of　different　paralleling　treatment　units,　which　is　calculated　from　the　water　level,　significantly　differ.　The　maximum　quantity　of　influent　water　is　1.72　times　the　minimum　quantity　of　influent　water.　Uneven　distribution　of　influent　wastewater　results　in　indistinct　difference　of　biological　treatment　effect.　The　effluent　NH4+-N　concentrations　of　the　bio-reactor　are　0.22-5.24　mg/L　and　0.60-0.85　mg/L　and　NH4+-N　removal　efficiencies　are　69.7%-97%　and　93.9%-95.4%,　respectively,　before　and　after　installing　the　water　quantity　adjustion　equipment.　The　water　quantity　is　balanced　and　effluent　quality　is　improved　due　to　the　equilibrium　of　water　quantity　through　the　adjustment　equipment.