Abstract: A highly accurate method for measuring serum sodium based on inductively coupled plasma mass spectrometry (ICP-MS) has been established in our laboratory. A specific amount of internal standard solution was added to a sample of serum sodium and to a series of sodium calibration solutions with different concentrations, then the solutions were nitrated with nitric acid and diluted with ultrapure water. The serum sodium concentration was calculated by the ratio of the sodium to the internal standard signal intensity. The measurement uncertainty of serum sodium by inductively coupled plasma mass spectrometry and the bracketing calibration method was evaluated according to Guide to the Expression of Uncertainty in Measurement (GUM) and the mathematical calibration model. The sources of uncertainty include bias, balance, repeatability measurements of the sample, density determination, and nonlinearity of the calibration model. After calculating the uncertainty of each component, all components were consolidated into combined standard uncertainty and an expanded uncertainty. This method provides a reference for the estimate of uncertainty when using similar methods and is of great significance to the standardization of clinical laboratories.