The sulfamethizole residues in organic fertilizers were determined according to“Determination of 19 veterinary medicines in organic fertilizer-Liquid chromatography-tandem mass spectrometry method”(GB/T40462—2021)，and the uncertainty was evaluated. The sourcesof uncertainty included the components of standard solutionpreparation，samplepreparation，measurementrepeatability，recovery and instrument. The calculation results of relative standard uncertainty indicated that the main factors affecting the uncertainty werethe fitting of the standardcurve，the purity of the standard andthe preparation process of the standardcurve，which should be focused and controlled during the actual experiment. It wasfound that at the 95% confidencelevel，the final result in the fertilizer sample was expressed asX(SMT) =(189.82±20.12)mg/kg when the expansion factor( k)was taken as 2.