A loading unit assembly used for a linear surgical stapler, which includes a staple cartridge frame and a knife driver beam. The knife driver beam has a movable safety block defined on a surface of a proximal end thereof, and a protrusion is correspondingly defined on the staple cartridge frame. When the loading unit assembly is at its initial position, a front surface at a distal end of the safety block engages with the protrusion to stop a movement of the knife driver beam and the safety block. When the loading unit assembly is at its working position, the safety block is relatively separated from the protrusion.
