Optics contamination has a great impact on X-ray transportation, especially in high-brightness, high-coherence X-ray free-electron lasers. However, there are very few related studies on the optics contamination of X-ray free-electron lasers. First, the characteristics of the X-ray free-electron laser are given. Then, the effects, control, and cleaning schemes of carbon contamination in synchrotron radiation are summarized. Moreover, the special effects of carbon contamination on the beam quality in X-ray free-electron laser beamlines are analyzed. Finally, the damage, melting, and blocking that particle contamination may occur on X-ray free-electron laser mirrors are analyzed, and the effects of the three conditions on the beam quality are given.