[Objective] To evaluate the uncertainty of protein content in pasteurized milk by Kjeldahl method. [Method] According to GB 5009.5—2016 National Food Safety Standard-Determination of Protein in Food,the Kjeldahl method was used to determine the protein content in pasteurized milk.Through the establishment of a mathematical model,the sources of various uncertainties in the experimental process were identified and quantified,and the relative standard uncertainty and expanded uncertainty of protein content in pasteurized milk were calculated. [Result] When the content of protein in pasteurized milk was 3.40 g/100 g,the expanded uncertainty of the measurement results was 0.02 g/100 g（k=2）. [Conclusion] The main sources of measurement uncertainty in this analysis method included the measurement repeatability,titration volume,and concentration of hydrochloric acid standard titration solution,providing scientific basis for quality control of protein content determination.