Using a Kjeldahl distillation apparatus, a method for the simultaneous determination of formaldehyde and sodium diacetate in aquatic products by dual-gradient liquid chromatography (DGLC) with online derivatization was established. Formaldehyde and sodium diacetate in samples were collected by steam distillation using the kjeldahl apparatus after adding phosphoric acid. The distillates were detectd by DGLC. In the left pump system, the content of sodium diacetate in aquatic products was directly detected. In the right pump system, 10 μL of acetonitrile-acetic acid solution containing 2,4-dinitrophenylhydrazine was used as the derivatization agent, and the sample volume was 10 μL. The content of formaldehyde in the distillate was detected after on-line derivatization for 4 min. The linear ranges of formaldehyde and sodium diacetate were 0.5–20.0 and 10–200 mg/L, respectively, with correlation coefficients greater than 0.999. The detection limits of formaldehyde and sodium diacetate were 0.5 and 10 mg/kg, respectively. For the intrabatch repeatability, the recoveries of formaldehyde and sodium diacetate were 81.4%–88.3% and 91.4%–102.9%, respectively, with relative standard deviations (RSDs) of 1.73%–2.68% and 3.08%–4.16%, respectively. For the interbatch repeatability, the recoveries of formaldehyde and sodium diacetate were 80.6%–87.9% and 90.8%–105.1%, respectively, with RSDs of 2.07%–2.72% and 3.44%–4.64%, respectively. The method is rapid and efficient, and is suitable for the simultaneous determination of formaldehyde and sodium diacetate in bulk samples of of aquatic products.