Hydrophobic ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate \[(C₈MIM) (PF₆)\] was synthesized using metathesis method, and then the ionic liquid was purified by activated carbon and macroporous resin solid phase extraction column, to get a colorless, transparent product \[(C₈MIM) (PF₆)\] with a yield of 87.6%. The ionic liquids was used to extract penicillin and penicillin-thiazole acid hydrolysates in milk and dairy products, and its extracting condition was optimized. After that the content of penicillin and penicillin thiazole acid were detected by utilizing self-developed HPLC method, and it could be indirectly determined whether the milk and dairy products were true antibiotic negative milk. In this study, the recovery rate of penicillin by ionic liquid extraction method was 87.9%～92.6%, with RSD 1.8%, and the detection limit was 5 ng/mL. Moreover, the recovery rate of penicillin thiazole acid was found to be 83.5%～90.8%, with RSD 2.4%, and the detection limit was also 5 ng/mL. Compared with both plate and liquid chromatography mass spectrometry methods, our method here is more predominant.