An optical element cleaning experiment with activated oxygen by synchrotron radiation (SR) is designed to research the cleaning process of carbon contaminated optical element surface. The thickness of sample carbon contamination layer is about 10.3 nm before cleaning process. Synchrotron radiation light is guided into a vacuum chamber which is filled with 1.0 Pa dry oxygen. The research shows that oxygen can be activited by SR and the graphite-like C on Si wafer can be removed effectively. By measuring the reflectivities of sample before and after cleaning process and comparing with IMD simulations,the C removal rate is about 1.75 nm/h.