Abstract: A method of gas chromatography-mass spectrometry(GC-MS/MS) for the determination of residues of picoxystrobin in water samples was set up, and the uncertainty of the method was evaluated according to the procedure of measuring uncertainty, and the source of uncertainty was analyzed. The components were calculated and synthesized. When the residual amount of picoxystrobin in water was 1.30 μg/L, its extended uncertainty was 0.08 μg/L. The main determinants of the uncertainty affecting the assay were the repeatability and accuracy of the method. This method provides a theoretical basis for the accuracy and confidence of the measurement of picoxystrobin residues.