The composite superabsorbent (CSA) with biodegradability was synthesized by using cassava starch mechanically activated for 60 min, acrylic acid and bentonite (BT) to enhance its water absorbency (Aeq) and retention. The effect of bentonite’s dosage on the water absorbency and the swelling properties of CSA were studied. The morphology, thermostability and functional groups of the samples were characterized by SEM, TGA and FTIR, respectively. The results showed that CSA had a microporous and laminate structure, the Aeq of CSA was increased from 1604 g/g to 1807 g/g by introducing an appropriate amount of BT (30 % to the weight of starch), and the thermostability and capacity of water-holding were also strengthened. The weight loss of CSA was 79.3 % when it was heated to 700 ℃, which was lower than that of superabsorbent without BT (83.4 %). The water retention for CSA was 48.3 % in the pressure of 1912 Pa and higher than that of superabsorbent without BT (45.5 %). Meanwhile, the swelling processes of CSA exhibited first-order dynamics, and the Aeq was of pH and salt dependent.