A pen barrel assembling device includes a base with a first frame and a second frame fixed to the top of two ends of the base. A movable rod extends through the first frame and fixed to the first frame by a first fixing bolt. An end part is connected to the first end of the movable rod and the outer section is positioned in the end part. A push rod extends through the second frame and is movable toward the movable rod by pivoting a lever. The metal section is connected to the free end of the push rod. Multiple blocks are pivotably or slidably connected to the base and each block includes an engaging recess which is removably engaged with the movable rod.