In this paper, a novel composite (C/CN-x) comprising biochar skeleton and "chrysanthemum" g-C3N4 was prepared by one-step co-thermal treatment of melamine and cellulose fiber precursors. The prepared composites were systematically analyzed, their visible light photocatalytic activity for the degradation of formaldehyde was studied, and a possible photocatalytic mechanism was proposed. C/CN-8 had the best photocatalytic activity, and the formaldehyde removal was 84.63%, 130% higher than that of the pure g-C3N4. "Chrysanthemum" g-C(3)N(4)increased the specific surface area and effective charge separation, and the biochar skeleton increased the absorption range of visible light and the local aggregation of formaldehyde, which was the key to the enhancement of photocatalytic activity. In addition to formaldehyde, the composite might be suitable for the degradation of other organic gas contaminants. The systematic analyses and possible photocatalytic mechanism provide a reference for removal of noxious pollutants. This work paves a way for using photocatalytic oxidation for the elimination of indoor air pollution.