The carbon contamination of extreme ultraviolet lithography system will degrade the reflectivity of multilayer. On the premise of guaranteeing the performances of optics，how to choose the carbon cleaning process is an important project. By analyzing the principle of various cleaning methods process theory, it is revealed that the influences of different technologies on the reflectivity of multilayer mainly are film oxidation, etching and surface roughness degradation. On the base of finite difference time domain and total integrated scattering theory, the relation between various influence factors and reflectivity of multilayer is studied. Results show that film oxidation and surface roughness degradation are main factors that result in decrease of the reflectivity of multilayer, while the influence for etching is minor. Based on the above analysis results, both radio frequency hydrogen plasma and atomic hydrogen cleaning technologies will not reduce the performances of optics and can be adopted as prior options, while the carbon contamination at the surface of optics is removed.