The invention relates to a printing mechanism which comprises a supporting device, a screen plate mounting device mounted on the supporting device and a mimeographing device slidably arranged on the screen plate mounting device. The supporting device comprises a support, a positioning assembly mounted on the support, a driving assembly and a plate lifting air cylinder; the screen plate mounting device comprises a supporting frame hinged to a driving assembly and connected with the plate lifting air cylinder, a first bearing arm, a second bearing arm and a third bearing arm which is slidably arranged on the supporting frame, wherein the first bearing arm and the second bearing arm are installed on the supporting frame; the mimeographing device comprises a base which is slidably mounted on the supporting frame and connected with the driving assembly, a driving air cylinder mounted on the base, a printing knife and an ink-returning knife, wherein the printing knife and the ink-returning knife are movably mounted on the base; and when the driving air cylinder conducts driving, the printing knife and the ink-returning knife are driven to make opposite vertical movement simultaneously. According to the printing mechanism, a printed workpiece is positioned through the positioning assembly to ensure that moving cannot occur, the printing quality is high, the printing working procedureand the ink-returning working procedure can be quickly completed through the printing knife and the ink-returning knife, and the printing efficiency is high.