Adjustable pen casing

The invention discloses an adjustable pen casing; the head portion and middle portion of the pen casing are screwed; the upper portion of the pen casing cavity is provided with a closed plastic box; a piston is arranged in the box; the top of a push rod is connected with the piston so as to push the piston, and the bottom of the push rod is connected with a pad; the pad makes contact with a pen core; the top of the plastic box is provided with an inflator inflating the box; the top of the inflator extends outside the top of the pen casing body; the pen casing is adjusted by the piston, so pen cores of different lengths can be installed.