Photocatalytic degradation can efficiently reduce the organic pollutants in wastewater which has broad application prospects. Herein, an algae Enteromorpha Prolifera, was used as the raw material to prepare alga-based carbon dots (CDs) through a hydrothermal method, and CDs-Cu-TiO₂ composite nanomaterials were further synthesized as a photocatalyst for pollutant degradation. The results show that CDs, Cu ²⁺ and TiO₂ are closely combined within the composite material. Compared to the pure TiO₂ nanoparticles, the absorption of CDs-Cu-TiO₂ in the visible light range is significantly enhanced and the fluorescence emission efficiency is decreased. Introduction of CDs and Cu²⁺produces a synergistic effect, which changes the band gap of the composite material to 2.35 eV, and effectively inhibits the recombination of electrons and holes. The photocatalytic degradation experiment using Rhodamine B as a model pollutant shows that under visible light irradiation the first order rate constant of degradation catalyzed by alga-based CDs-Cu-TiO₂ reaches 6.4 times of that degraded by pure TiO₂ nanoparticles, and the degradation efficiency after 150 min is close to 100%, which is double of the value when TiO₂ nanoparticles apply.