The invention provides a derivatization method for detecting 3,5,6-trichloro-2-pyridol by gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS/MS). The method comprises the following steps: processing a test liquid of 3,5,6-trichloro-2-pyridol by the use of a desolventizing agent, dissolving by the use of acetone (for pesticide residue), uniformly mixing, centrifuging to remove solid impurities, and carrying out desolventizing treatment; respectively adding acetic anhydride (analytically pure) and ethyl acetate (for pesticide residue), uniformly mixing, reacting in water bath of 50-60 DEG C for 30-50 min, and standing at room temperature for 3-5 min; adding acetone (for pesticide residue), and uniformly mixing repeatedly to obtain a sample to be tested; detecting the sample to be tested by GC or GC-MS, controlling starting temperature of GC to 100 DEG C, heating to 150 DEG C within 2 min, keeping for 2 min, continuously heating to 280 DEG C, and keeping for 5 min; and carrying out qualitative and quantitative determination by selecting at least two characteristic fragment ions through tandem mass spectrometry multiple-reaction monitoring. The invention has the following advantages: cost of selected raw materials is low, and reaction condition is mild.