Over 2 million tons of citric acid residue (CAR) are generated from citric acid production in China each year. Thus, it is of great significance to find a suitable method for recycling the waste products of citric acid fermentation. Herein, activated carbon (AC) samples were prepared from CAR with phosphoric acid activation. Effects of impregnation ratio, activation time, and activation temperature on pore structure and surface functional group formation were evaluated. AC prepared by 1.5 h of pyrolysis in nitrogen at 550°C from CAR and phosphoric acid at an impregnation ratio of 2.0 (CAR-AC-2.0) had the largest specific surface area (786 m ² /g) and total pore volume (0.71 mL/g). The surface morphologies and chemical properties of AC samples were characterized by scanning electron microscopy, thermogravimetric analysis, elemental analysis, and Fourier transform infrared spectroscopy (FTIR), and their adsorption capacities for methylene blue (MB) and iodine were tested. There were rare functional groups on the surfaces of CAR-AC-2.0, and the adsorption amounts of CAR-AC-2.0 for iodine and MB reached up to 729 and 121 mg/g, respectively. CAR-AC-2.0 was an ideal adsorbent for removing organic matters from solutions and wastewater. Adsorption efficiency of CAR-AC-2.0 for chemical oxygen demand (COD) from sugar-containing wastewater was 291.6 mg/g, and the removal efficiency was 80.9% when the dosage was 15 g/L. This study provided a new strategy for removing COD from fermentation wastewater with high efficiency and low cost.