An adjusting-fixing assembly for a staple gun has a sliding frame, an adjusting nut, a fixing arm and an abutting shaft. The sliding frame is movably connected to the staple gun and has a connecting board with two positioning recesses. The adjusting nut is rotatably mounted on the sliding frame and has an engaging segment. The engaging segment is formed on the adjusting nut between the threaded end and the connecting board and has multiple engaging faces. The fixing arm is rotatably connected to the sliding frame, engages with the adjusting nut and has a connecting rod, an engaging recess and a positioning element. The engaging recess is formed in an inner side of the fixing nut and engages with two adjacent engaging faces of the engaging segment. The abutting shaft is connected to the adjusting nut and is movably connected to the staple gun.