Coupling graphitic carbon nitride (CN) with carbonaceous materials is an effective strategy to improve photocatalytic performance, but the contributions of carbonaceous materials are not fully understood. Herein, a new type of carbon/CN (CCN) complex photocatalyst is synthesized with a 6-fold enhancement of H₂ evolution rate compared to that of pristine CN. The role of carbon in photocatalytic H₂ evolution reaction is systemically studied and it is experimentally and theoretically revealed that carbon mainly contributes to the improved capability of exciton dissociation and enhanced electric conductivity for charge transfer, leading to an increased population of photo-carriers for photocatalytic reactions. Interestingly, the enhanced light absorption originated from carbon barely generates charge carriers for H₂ evolution activity. These new findings will inspire the rational design of carbon-based photocatalysts for efficient solar fuel production.