Abstract: Lead (Pb²⁺) and chromium (Cr³⁺) are common heavy metal pollutants widely present in electroplating, mining, and industrial wastewater, posing serious threats to the ecological environment and human health. Traditional treatment methods face challenges such as high costs, secondary pollution, and inefficiency, making the development of an efficient, economical, and environmentally friendly heavy metal removal technology essential. Adsorption has become an important alternative technique due to its simple operation, low cost, and lack of secondary pollution. Phosphorus-magnesium-silicon doped biochar composite material (Mg-HAp-BC) was prepared by incorporating magnesium oxide (MgO) and hydroxyapatite (HAp), significantly enhancing the stability and adsorption performance of biochar in acidic environments. A response surface methodology was used to optimize factors such as liquid-solid ratio (L/S), pH value, and doping ratio, and their effects on the adsorption performance of Mg-HAp-BC were systematically studied. Experimental results indicate that under a liquid-solid ratio of 0.05 and the optimal doping mass ratio (m(MgO)∶m(HAp) = 6.10∶7.99), the composite material exhibits significant advantages in removing Pb²⁺ and Cr³⁺, especially under low pH conditions, where the adsorption rate is rapid, exhibiting monolayer uniform adsorption. At higher pH values, the adsorption process becomes more complex, involving multilayer non-uniform adsorption. Adsorption mechanism analysis shows that the adsorption performance of Mg-HAp-BC composite material is mainly dependent on electrostatic adsorption, surface mineral modification, π—π interactions, and pore filling. Additionally, ion exchange reactions may occur during Cr³⁺ adsorption. In conclusion, Mg-HAp-BC composite material demonstrates excellent adsorption performance under various pH conditions and shows promising potential in heavy metal pollution remediation, with broad environmental adaptability and practical application prospects.