Abstract: The S44 DN200 valve was leaked during operation because of the seat retainer corrosion. By means of chemical composition analysis, metallographic examination, pitting corrosion and so on, the corrosion reason of seat retainer was analyzed. The results showed that the PREN of retainer was lower than standard, the heat treatment process was not according to the specification, the seat retainer corrosion led to the valve leaked in seawater containing high concentrations of chloride.