An injection pen comprises a starting mechanism (1) and a driving mechanism (2). A needle pin (40) of the starting mechanism (1) fitted to a connecting rod (30) and a front spring (60) is disposed in a front housing (10) and an inner casing (20) therein. A front cover (50) is disposed at the front end of the front housing (10) and covers the needle pin (40). A rear housing (70) of the driving mechanism (2) and a driving casing (80) therein are connected to the front housing (10). A trigger tube (90) and a key panel (A0) are mounted in the driving casing (80). A push rod (A1) fitted to a rear spring (A2) is mounted in the trigger tube (90). An elastic baffle (83) of the driving casing (80) fastens the driving casing (80) and the push Rod (A1) and locks the rear spring (A2). An injection needle group (3) can be mounted between the starting mechanism (1) and the driving mechanism (2). When the front cover (50) is removed, the needle pin (40) can be pressed and inwards retreated into the front housing (10) to make a needle (3A) to be exposed, and the inwards retreated needle pin (40) drives the connecting rod (30) to push away the elastic baffle (83) of the driving casing (80) to perform unlocking. The released rear spring (A2) pushes the push rod (A1) to make an injection needle group (3) to transfer a drug. After the injection, the needle pin (40) is reset under the push of the front spring (60), and the needle pin (40) is fastened in the inner casing (20) when a key panel (A0) is pulled out by means of the connecting rod (30), so that the needle pin (40) cannot be pressed to inwards retreat, and accordingly, a used syringe needle (3A) is protected, so as to prevent users from being hurt by the needle by accident.
