Based on“method for the determination of multiple residues of prohibited pesticides in medicinal materials and herbal slices（Plants）” in Chinese Pharmacopoeia（2020 edition），the uncertainty in determination of benzene dichloro-diphenyl-trichloroethane（DDT）residues in Baishao（Paeoniae Radix Alba）was evaluated by gas chromatography-tandem mass spectrometry（GC-MS/MS）was carried out. The measurement mathematical model was established. Source analysis including preparation of standard solution，sample preparation，measurement repeatability，standard curve，recovery and GC-MS/MS. By calculating and synthesizing each uncertainty component，the measured result of 4，4’-DDE，2，4’-DDT，4，4’-DDD and 4，4’-DDT can be respectively reported as（0.14±0.01），（0.13±0.01），（0.14±0.01），（0.13±0.03）mg·kg^-1（k=2）. The uncertainty of the determination result was mainly affected by the uncertainty of standard curve，standard solution preparation.