The surface of polytetrafluoroethylene (PTFE) is modified by the irradiation from a focused excimer laser with wavelength of 248 nm with different energy densities. Scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy are used to investigate the changes of the surface morphology, chemical composition and structure. The mechanism of interaction between laser and PTFE is further studied. The experimental results indicate that the component of fluorin reduces greatly after laser irradiation and there appear the carbonization of the surface, cross-linking and the formation of oxygen function group. When the laser energy density is increased, C=C bond is produced. These changes of structure can enhance the hardness and adhesion of the surface. The physical properties and chemical structure of the treated samples surface are influenced greatly by the laser energy density, which is an important factor in the process of laser modification and ablation.