Introduction  The ultra-supercritical unit is the main power generation unit in China， which has strict requirements on water steam quality. In this paper， water steam quality control measures have been proposed to prevent scaling and corrosion of the thermal system and ensure safe and economic operation of the unit.

  Method  To control the steam-water quality of ultra-supercritical units， proper demineralized water production process， reliable condensate polishing system， feedwater oxygenation treatment （OT）， reasonable on-line monitoring meter， appropriate condenser pipe and shutdown protection were selected.

  Result  The above measures can effectively prevent scaling and corrosion of the thermal system and ensure the safe and economic operation of the unit.

  Conclusion  The steam-water quality control of ultra-supercritical parameters units can refer to the Water Vapor Quality of Thermal Power Units and Steam Power Equipment （GB/T 12145—2016）， and it is recommended to control the Parameters according to the expected value.