This standard specifies the water quality requirements for seawater chilled down, and the method for determining the content of calcium and mg ions using the EDTA complexed Titrimetric method. It is suitable for seawater chilled down with calcium content between 200 mg/L and 1,000 mg/L and mg content between 800 mg/L and 3,200 mg/L. The standard details the reagents, instruments and detection steps required for analysis, providing a scientific basis for the monitoring of calcium and mg ion content in seawater chilling down systems, which helps to ensure the normal operation and prolong the service life of seawater chilling down equipment.