The resource utilization of sheep dung from farming waste was evaluated in adsorption of total phosphorus (TP). Four types of modified sheep dung activated carbon (SDAC) were prepared: SDAC without metal (SDAC-N), SDAC with lanthanum (SDAC-La), SDAC with magnesium (SDAC-Mg), and SDAC with La and Mg (SDAC-La-Mg). The various SDACs prepared were tested in the adsorption TP from wastewater. The main components of the SDAC-La-Mg surface were MgO and La2O3. The SDAC-La-Mg surface was rich in functional groups such as CC and CO and acquired a negative charge. At 25 °C and pH of 4, the TP adsorption capacity of SDAC-La-Mg was 1.94 mg/g. Among the four SDAC adsorbents, SDAC-La-Mg exhibited the highest adsorption capacity. TP adsorption by SDAC-La-Mg followed the secondary kinetic and Langmuir isothermal adsorption models, primarily through chemisorption mechanisms such as ion exchange, chemical precipitation, and covalent bonding. SDAC-La-Mg was regenerated, and the TP removal rate remained at 65% after three regeneration cycles. Mg and La ions combine with carboxylate ions to form ligand compounds, enabling TP adsorption by displacing the -OH group on the surface of SDAC-La-Mg to form an inner complex. Thus, SDAC-La-Mg can be used as an effective adsorbent for TP in wastewater treatment.