Ferric chloride (FeCl3) modified reed straw-based biochar was synthesized to remove nitrate from aqueous solutions and achieve waste recycling. The adsorption of nitrate onto Fe-RBC-600 adsorbents could be described by the pseudo-second-order kinetic model and fitted to Langmuir adsorption, and the maximum adsorption capacity predicted using the Langmuir model was 272.024 mg g−1. The adsorbent characterization indicated that a high temperature of 600 °C and an oxygen-poor environment could develop a hydrophobic surface and O-containing functional groups on the biochar, which provided more binding sites for Fe3+/Fe2+ attachment and increased the surface functionality of Fe-RBC-600 with iron oxide formation. The increasing surface functionality successfully enhanced the nitrate adsorption property. The mechanism of nitrate adsorption was mainly attributed to the physical adsorption onto the positive surface and sequential chemical reduction by Fe2+, and the electrostatic adsorption by protonated amine groups.