Abstract: In order to make better decisions when the concentration of chlorpyrifos was located near the maximum residue limit. This study combined with the specific operation steps of our own laboratory, referred to cnas-gl 006, metrological verification (technical) specifications and other relevant standards, according to "GB 23200.113-2018 determination of 208 pesticides and metabolites residues in foods of plant origin", the uncertainties of chlorpyrifos residues in cucumber was evaluated by gas chromatography-tandem mass spectrometry(GC-MS/MS) combined with external standard quantitative method. The results showed that: at the level of 0.10 mg/kg and 95% confidence interval (k=2), the determination results of chlorpyrifos could be expressed as: (0.10 ± 0.01）mg/kg;The uncertainty component of standard solution preparation has the highest contribution to the uncertainty of chlorpyrifos. It could not be determined as unqualified when the residual amount of chlorpyrifos in cucumber ≥0.1 mg/kg and ≤ 0.11 mg/kg. It is to use the volumetric flask with smaller error,reduce the reading error of the operator,reduce the number of pipettes,and reduce the temperature difference of the environment,and then perform the measurement again. The uncertainty assessment results will minimize the possible disputes in the judging of the residue values near the maximum residue limits.