The adsorption performance of layered double hydroxides (LDHs) is limited owing to self-aggregation. To avoid this and effectively control the eutrophication of water bodies, biochar (BC) was synthesized, herein, by pyrolyzing waste sheep manure at 500°C, and Ca–Al-LDHs were loaded on the surface via a coprecipitation method to obtain Ca–Al-LDHs–BC(CA) composites with varying LDH contents. The fitted maximum adsorption capacities of the CA-5%, CA-10%, CA-15%, and CA-20% samples (corresponding to samples with 5%, 10%, 15%, and 20% LDHs, respectively) were 10.21, 16.14, 22.40, and 28.47 mg g−1, which were (when converted into metal proportions) 1.48, 1.23, 1.15, and 1.13 times of that of single hydrotalcite, respectively. The double-layer model was fitted using the Levenberg–Marquardt iterative algorithm, which when combined with the characterization results, confirmed that the adsorption of phosphate ions by CA–BC occurred via the double-layer adsorption mechanism. Two types of direct adsorption were observed: ion exchange, which resulted in first-layer adsorption, and ligand exchange, which resulted in second-layer adsorption, with first-layer adsorption accounting for a higher proportion. This double-layer adsorption mechanism showed that LDHs–BC could achieve higher ligand exchange performance compared to that achieved using only LDHs.