This paper focuses on the reflectivity decrease of reflective elements caused by the carbon contamination deposited on multilayers during EUV( Extreme Ultraviolet) lithography working and emphasizes the clear method of carbon contamination deposited on multilayers. The process of carbon contamination deposited on multilayers was elucidated and the damage of carbon contamination on the multilayers was introduced briefly. Several kinds of cleaning methods for carbon contamination deposited on multilayers were described in detail from cleaning mechanism， removing rate and cleaning effect， and their advantages and disadvantages were analyzed. The result indicates that the cleaning rates of plasma oxygen and activated oxygen reach 2 nm/min， but the multilayer surface is easy to be oxidized in cleaning process; and the plasma hydrogen and atomic hydrogen have relatively slow cleaning rates， they are only about 0.37 nm/min， but the multilayer surface is hardly to be oxidized. Moreover， the difficulties of different cleaning methods for X-ray multilayer mirrors in-situ are discussed.