In order to analyze and evaluate the uncertainty of determination of famoxadone pesticide residues in grape by UPLC-tandem mass spectrometry, the pesticide famoxadone was selected to evaluate the measurement uncertainty of the methods from six aspects including sample weigh, standard solution preparation and standard curve fitting, etc., with "National food safety standard-Determination of 331 pesticides and metabolites residues in foods of plant origin-Liquid chromatography-tandem mass sprctrometry method" (GB 23200.121-2021) and "Determination of 450 pesticides and related chemicals residues in fruits and vegetables-LC-MS-MS method" (GB/T 20769-2008) standard methods as the research objects. The results showed that the relative extended uncertainty of famoxadone pesticide residue was about 0.22% (k=2), in which the main uncertainties were derived from the use of UPLC-tandem mass spectrometer, standard solution preparation, standard curve fitting and standard recovery rate. In order to reduce the uncertainty of test results, in actual work, we need to improve the stability of equipment, optimize the preparation process of standard solution and improve the operating skills of test personnel. At the same time, we should also take the results of uncertainty as an important reference to judge whether the limit threshold data meets the requirements of the limit standard.