The invention discloses a pretreatment kit and method for detecting niclosamide in aquatic products. The kit comprises a kit body and reagent bottles which are respectively put in six empty areas of a, b, c, d, e and f in the kit body. The method comprises the following steps: A. putting a sample into a centrifugal tube, adding a first reagent, and oscillating; B. adding a second reagent, oscillating, centrifuging, and transferring supernate; C. adding the first reagent into the original centrifugal tube, oscillating, after centrifugation, taking the supernate out and mixing the supernate with the last supernate; D. metering the mixed liquid with a certain volume, after shaking uniformly, taking certain volume of liquid and putting into the centrifugal tube, and blow-drying with nitrogen; and E. dissolving residues with the solution of a third reagent, adding a fourth reagent, oscillating, centrifuging, taking the clear liquid and filtering, and carrying out HPLC-MS/MS(high performance liquid chromatography-mass spectrometry/mass spectrometry) measurement. The detecting method is simple, fast and accurate, high in sensitivity and low in cost, and is easy to popularize. The pretreatment kit is suitable for treatment of eatable samples of large amount of fresh and alive and ice aquatic products, and the pretreatment efficiency of the samples is greatly improved.