Abstract: A new method was proposed to preparation of activated carbon layer on SiO_2f/SiO₂ composite to assist brazing alloy wetting, in order to solve the problems that poor wettability of brazing alloy on SiO_2f/SiO₂ and poor performance of joints. PECVD was occupied to preparation of extremely thin activated carbon layer on the surface of SiO_2f/SiO₂ composite to assist active brazing alloy spreading and wetting. Analysis showed that the method can prepare activated carbon layer on the surface of SiO_2f/SiO₂ composite at the temperature of 400 ℃ and holding time for 15 min, and the surface morphology of composite showed no obvious changes. Wetting results showed that introducing activate carbon layer can significantly improve the wettability of Ag-Cu-Ti brazing alloy on the surface of SiO_2f/SiO₂ composite when the wetting angle dropped from 131° to 27° at the temperature of 860 ℃ and holding time for 10 min. Activate carbon layer can promote the wettability of active brazing alloy, because carbon showed good affinity with active elements to activate the surface state of SiO_2f/SiO₂ composite.