Inductively coupled plasma optical emission spectrometry was used to determine the potassium content in soil geochemical samples(NST-4，Sichuan purple soil)，and the uncertainty of the measurement results was evaluated and expressed.Analyzed the sources of uncertainty in the testing process，including sample weighing，standard solution preparation，calibration curve fitting，glass measuring tools，instruments and test repeatability components，and calculated the relative standard uncertainty of each component，and it was found that the uncertainty of measurement result was most affected by the dilution and preparation of the standard curve.Evaluation according to GB/T 27418-2017 "Evaluation and expression of uncertainty in measurement"，the expanded uncertainty of measurement for soil samples(NST-4，Sichuan purple soil)with potassium content of 20.2 g/kg was 0.7 g/kg(a coverage factor k=2，at probability 95% level).