A retractable pen includes an improved triggering device (i.e., an actuating means), which is easy for manufacturing and user-friendly. The retractable pen has a pen-body having two pen-tubes, a pen-cartridge within the pen-body, and an actuating means. The actuating means not only triggers the movement of pen-cartridge (slidable protrusion/retraction of the pen-cartridge) but also, at the meantime, triggers length-change of pen-body (slidable movement between the two pen-tubes).