The classic Cassegrain system combined with a front lens group is chosen and a high-power laser-beam focus system over a variable long distance is designed based on the primary aberration theory. When the moving distance of the target objects is within 500-5000 m, it is achievable to control the focusing spot size by the adjustment of the distance between the front lens group and the Cassegrain system and the different object distances can be reversed from the location of the lens group. The initial structure is optimized with the Zemax optical software, and the design results show that the spot sizes under different object distances all meet the design requirements.