A hermetically-sealed formaldehyde extractor comprises: 1. an electric heating sleeve, 2. a round-bottom distilling flask, 3. No.1 sand core filter plate, 4. an extracting part, 5. a sintering sealed division board, 6. a condensing part, 7. a condensed water outlet, 8. a ground glass plug, 9. a pressure balancing tube, 10. a condensed water inlet, 11. a steam liquid outlet pipe, 12. an absorption liquid outlet, and 13. a distillation and siphon pipe. The operation method comprises the following steps: weighing 105 to 110 grams of a sample, putting the sample into the round-bottom distilling flask 2, adding 600 mL of toluene into the distilling flask, opening the ground glass plug 8 in the extractor, adding 300 mL of water extract liquid, covering the ground glass plug 8, starting the cooling circulation water, starting to heat, heating until the toluene reaches the boiling point, then carry out refluxing for 2 hours, in this period, cooling the evaporated mixed liquid of toluene and formaldehyde in the condensing part 6, subjecting the condensed liquid to go through the extracting part 4, then subjecting the extract residual liquid to go through the distillation and siphon pipe and reflux to the round-bottom distilling bottom 2, keeping on the continuous reflux of toluene, distilling for 2 hours, washing the extract liquid into a 1000 mL volumetric flask through the absorption liquid outlet 12, and finally diluting the extract liquid to 1000 mL for later detection.