Slotted verticality of the die-board laser cutting is studied by using the NEL-A series high-power fast-flow axis CO2 laser and the SLCM-1225 numerical control machine. Based on geometric optics theory, possible paths of the incident light on the convex lens are discussed. The result shows that the small deviation of the optical path is the main cause of the non-vertical slot, and a veiw is given that the verticality of the slot can be improved by shifting the convex lens. The testing results show that the horizontal movement of the convex lens can change the position of the energy center lines of the laser beam, thus change the direction of the the energy center lines of the focused beam, and then change the angle between the slot and the board surface.