Dynamic focal laser marking machines usually use a collimating and beam expanding system before the focusing lens. The laser beam will always focus in the marking plane by adjusting the motion of collimating and beam expanding system according to the galvanometer scanning. But for a larger marking area, the effect of focusing is not so ideal and even out of focus at the edge. In order to satisfy large area marking, a dynamic collimating and beam expanding system is designed based on normal expander system. The focusing performance is improved by 20% by optimizing the trajectory of collimating and beam expanding system. It achieves the target of small spot size in bigger area. Using polynomial fitting to calculate the beam expander′s trajectory, the marking speed is improved. Piecewise linear interpolation look-up table is used to correct the pincushion distortion at the edge. It is shown that the machine works pretty well and conforms to specified requirements.