Abstract: pX measurement is usually carried out by laboratory ion meter based on the principle of electrochemical analysis, and the working curve needs to be calibrated with a standard solution before measurement. Since the prepared value of the standard solution for calibration is the concentration value and the instrument response quantity is the activity value, and activity and concentration are two different quantities, the unmeasurable and difference of the activity coefficient make the pX measurement have a large measurement error. In this paper, a processing method of the activity coefficient in the uncertainty assessment of the pX measurement error is discussed, taking the data of one experiment as an example.