An anti-theft lock for a portable electronic device has a mounting shaft, two buckling units, a split-abutting unit, and a controlling unit. The two buckling units are moveably mounted around the mounting shaft. The split-abutting unit is moveable and has an engaging end and a split-abutting end. The split-abutting end selectively abuts the two buckling units via the movement of the split-abutting unit. The controlling unit has an annular curved surface. The annular curved surface slidably abuts the engaging end of the split-abutting unit. During the rotation of the controlling unit, the annular curved surface is capable of pushing the split-abutting unit to make the split-abutting end abut the two buckling units with inclined surfaces to move the two buckling units away from each other. The larger a distance the split-abutting unit moves, the larger a distance the two buckling units move away from each other.