This standard specifies a method for measuring the water activity of grain grains by instrumentation. It is suitable for grain grains with a water activity range of 0.030 to 1.00. Water activity refers to the ratio of the saturated vapor pressure of moisture in grain grains to the saturated vapor pressure of pure water at the same temperature. The standard describes that in a closed water activity meter sample measurement chamber, the free moisture in grain grains reaches equilibrate through humidity sensors or digitalization probes. The measured relative humidity is the water activity. The reagents and materials used must meet the analytical purity standard, including heavy distillation water or water of equivalent purity, and saline solutions at specific concentrations. The standard ensures the accuracy and reliability of the determination of grain moisture activity.