<p>Remediation of eutrophic water is a challenging task. Modification of biochar has been demonstrated to enhance the removal ability of contamination of biochar. In this work, magnesium (Mg) and copper (Cu) were incorporated in biochar during preparation to increase the removal of phosphate (P) and inhibit algae bloom. The results showed that Mg-modification significantly (<i>p</i> &lt; 0.05) improved the adsorption of phosphate, and the adsorption capacity was 138.7 mg g⁻¹, and the improvement on P removal was attributed by the formation of MgO which enhanced the sorption affinity on PO₄³⁻. In addition, Cu-modification showed significant (<i>p</i> &lt; 0.05) inhibition effect on growth of cyanobacteria as compared to original biochar with biomass decrease of 62% for Cu-biochar and 85% for Cu-Mg-biochar after three week incubation. Among all modified biochar composites, Cu-Mg-biochar showed highest inhibition effect due to the removal of nutrient and directly causing cell lysis. Moreover, in the water co-existing with different ratios of oil and PO₄³⁻, Cu-Mg-biochar showed highest (<i>p</i> &lt; 0.05) efficiency in inhibition on the growth of toxic cyanobacteria, biomass decreased from 2.85 mg L⁻¹ to 0.20 mg L⁻¹. Overall, Cu-Mg-biochar composite was suggested effective remediation material in treating oil-impacted eutrophic water.</p> Graphical Abstract <p></p>