The use of functional materials such as carbon-bismuth oxyhalides in integrated photorefineries for the clean production of fine chemicals requires restructuring. A facile biomass-assisted solvothermal fabrication of carbon/bismuth oxychloride nanocomposites (C/BiOCl) was achieved at various temperatures. Compared with BiOCl and C/BiOCl-120, C/BiOCl-180 exhibited higher crystallinity, wider visible light absorption, and a faster migration/separation rate of photoinduced carriers. For the selective C–C bond cleavage of biomass-based feedstocks photocatalyzed by C/BiOCl-180, the xylose conversion and lactic acid yield were 100% and 92.5%, respectively. C/BiOCl-180 efficiently converted different biomass-based monosaccharides to lactic acid, and the efficiency of pentoses was higher than that of hexoses. Moreover, lactic acid synthesis was favored by all active radicals including superoxide ion (·O₂⁻), holes (h⁺), hydroxyl radical (·OH), and singlet oxygen (¹O₂), with ·O₂⁻ playing a key role. The fabricated photocatalyst was stable, economical, and recyclable. The use of biomass-derived monosaccharides for the clean production of lactic acid via the C/BiOCl-180 photocatalyst has opened new research horizons for the investigation and application of C–C bond cleavage in biomass-based feedstocks.