This standard specifies the detection method of chloride ion diffusion coefficient of cement sand, including equipment, material requirements, laboratory conditions, test body forming and curing, detection of chloride ion diffusion coefficient, result processing, detection deviation and performance evaluation. It is suitable for general-purpose silicate cement, marine silicate cement and Miscellaneous designated cement. The detection method includes standing the test body under negative pressure of vacuum salt-saturated equipment and soaking it in sodium chloride solution for 18 hours. After the test body is dried, the chloride ion erosion performance of cement sand is evaluated by measuring the conductivity of the test body and calculating the chloride ion diffusion coefficient according to the Nernst-Einstein equation. This method provides a scientific basis for evaluating the durability of cement materials in practical applications.