<p>Methylene blue is a significant pollutant that seriously threatens the safety of water environments, and traditional methods have difficulty removing it. In this study, lignin carbon quantum dots (CQDs) were prepared by the hydrothermal method, and a BiVO₄/CQD composite material was fabricated for the photocatalytic degradation of methylene blue. Through various testing methods, the optimal composite with carbon quantum dots was successfully studied. The incorporation of carbon quantum dots reduced the band gap of BiVO₄ and enhanced the light absorption and electron transfer ability of the BiVO₄/CQD composite material. The incorporation of CQDs can effectively enhance the degradation performance of BiVO₄/CQD composite material toward methylene blue. Under illumination for 120 min, the degradation rate of methylene blue by pure BiVO₄ material was only 56.2%, while the degradation rate of methyl blue by BiVO₄/CQDs-15 composite material could reach as high as 99.7%. The enhancement of photocatalytic activity is attributed to the highly dispersed nature of CQDs on the surface of BiVO₄, their upconversion property, excellent electron transfer ability, and the synergistic effect on the photocatalytic performance of BiVO₄.</p>