High-voltage and ultra-high-voltage transmission cables are exposed to natural environment all the year round. Plastic and other non-metallic foreign bodies are often overlapped on cables, towers, insulators and other power grid facility, which leads to poor insulation among high-voltage power cables, and becomes one of the major threats to the normal operation of power systems. As a new light source, laser has a good application prospect in the removal of non-metallic foreign bodies such as polymers because of its high energy, strong monochromatic and good coherence. The use of laser to remove non-metallic foreign bodies such as plastics winding on transmission lines has the advantages of fastness, high efficiency, safety, so it has been widely used in the removal of foreign bodies in high-voltage and ultra-high voltage power lines. In this paper, the mechanism of laser ablation is analyzed and the optical system of laser ablation unit is designed. According to the actual situation of laser ablation of polymer for transmission line, the diameter of laser spot at 10-180m is determined to be 32mm. In the consideration of the error factors, the imaging quality of the optical focusing unit for the designed optical system is simulated. At the same time, the far-field characteristics of the laser are simulated and studied. The results show that the range of RMS SPOT radius change is less than the diffraction limit radius by sensitivity analysis of laser ablation unit considering errors. The designed optical focusing unit has good imaging quality and meets the requirements. With the increase of laser transmission distance, the diameter of laser spot increases slightly. The maximum spot diameter at 180m is 29mm, which is less than the design value. The designed optical system can meet the needs for laser ablation of polymer and has good effect.