Abstract: In order to improve accuracy of potato food safety detection results, high performance liquid chromatography-tandem mass spectrometry was used to determine methomyl in potato, analyze specific experimental process, establish a mathematical model, determine and compare components of measurement uncertainty．When residue of aphid phosphorus in potato was 1.3 μg/kg, expanded uncertainty was 0.84 μg/kg（k=2）．Results showed that among measurement uncertainty components, uncertainty caused by instrument itself, standard curve drawing and experimental repeatability were main factors．