An injection pen has an actuating mechanism (1) and a driving mechanism (2). The actuating mechanism (1) has a needle hub (40), a connection rod (30), and a front spring (50) mounted in a front casing (10) and an inner casing (20). A front cap (50) is mounted on the front casing (10) and is mounted around the needle hub (40). The driving mechanism (2) has a rear casing (70) and a driving casing (80). A driving tube (90) and two engaging members (A0) are mounted in the driving casing (80). A pushing rod (A1) connected with a rear spring (A2) is mounted in the driving tube (90). Resilient tabs (83) on the driving casing (80) are engaged with the connection rod (30) to lock the rear spring (A2). A syringe can be enclosed by the needle hub (40) to prevent an accidental needle stick.
