Abstract: OBJECTIVE To study the amount of silicone oil used on the surface of rubber plunger of prefilled syringe and the migration of silicone oil into methotrexate injection. METHODS The content of silicone oil on the surface of brominated butyl rubber plunger in pre potted syringe was determined by Fourier transform infrared spectroscopy, the migration amount of silicone oil in methotrexate injection was determined by inductively coupled plasma mass spectrometry. RESULTS The content of silicone oil on the surface of plunger was each 0.191 mg, the maximum migration amounts of 0.5, 0.2, 0.15 mL methotrexate injection were 2.65, 1.26, 1.04 μg, respectively. CONCLUSION Fourier transform infrared spectroscopy and inductively coupled plasma mass spectrometry can be used to determine the amount of silicone oil on the surface of rubber plunger and the migration of silicone oil into the preparation.