Direct flow steam generator nuclear power unit, such as high-temperature gas cooled reactors, sodium cooled fast reactors, and small reactors, are developing rapidly. Their characteristics are high steam parameters and special evaporator structures, which require strict control of feedwater quality indicators. The quality control of water and steam in the unit is a long-term and complex systematic work. This article first analyzes the characteristics of the direct flow evaporator unit, draws on the operating experience of high-temperature reactors and sodium cooled fast reactors, and proposes six water quality control measures from the construction period, commissioning period, and operation period. During the infrastructure construction period, strict anti-corrosion maintenance measures should be implemented for equipment and pipelines during the storage phase to reduce the generation of corrosion products. Clean installation processes should be implemented during the infrastructure installation period to reduce the entry of foreign objects such as weld beads and slag into the thermal system; During the debugging period, thoroughly remove corrosion products and welding slag from the feedwater system and steam pipes through chemical cleaning of the secondary thermal system, and adopt reasonable maintenance processes to maintain the system and equipment to prevent secondary corrosion; There are two ways to control water quality during operation: the fine treatment system is the only purification method for the thermal system. Firstly, by adjusting the operation mode of the fine treatment system, the water quality of the secondary circuit can be controlled; In addition, adding oxygen to the feedwater is an effective method to reduce the FAC of the secondary circuit, and the water quality can be improved by adding oxygen to the feedwater in the secondary circuit. Most of the methods and processes mentioned above have been applied and achieved good results, which have high reference value for other units of this type.