In order to develop a method for the determination of trace pentachlorophenol and its sodium salt in animal-origin foods by automated solid phase extraction-coupled with ultra performance liquid chromatography-tandem mass spectrometry, the effects of extractive solvents, solid phase extraction columns, eluents, mass spectrum and chromatographic separation conditions, internal standards were studied and compared. Finally, the sample was extracted by 5% ammonia acetonitrile, purified by MAX solid phase extraction column and eluted by 10% formic acid methanol. The extract was dried-up with nitrogen and re-dissolved with acetonitrile water solution (5050)．The solution was separated on a Kinetex C₁₈ column (100 mm×2.1 mm, 2.6 μm) with gradient elution program of acetonitrile and 5mmol/L ammonium acetate (0.1% formic acid) as the mobile phase. The detection was performed by UPLC-MS/MS using the negative electrospray ionization interface under the multiple reaction monitoring mode (MRM), and quantified with internal standard method. The developed method showed a good linearity in the range of 0.2~10.0 μg/L, with correlation coefficients (r) above 0.999. The mean recoveries spiked at levels of 1.0, 5.0, 10.0 μg/kg were within 86.0%~106.0%, with relative standard deviations (RSD) were in the range of 1.62%~4.27% (n=6). The method is sensitive, reproducible, accurate and precise, and is suitable for the determination of trace pentachlorophenol and its sodium salt residues in animal-origin foods.