The purpose of this study was to use ultrasound-based extraction to prepare starch from the tubers of Cyperus esculentus. Ultrasonic treatment of Cyperus esculentus powder with a medium of alkaline-treated water can effectively improve the starch extraction efficiency. Box-Behnken design was used to optimize the extraction process, and the results showed that the optimal parameters were ultrasound time of 30 minutes, pH value of 9.0, ultrasound temperature of 40 °C, and solid-liquid ratio of 10:1. The extraction percentage under these conditions was 90.1%. The physicochemical properties of C. esculentus starch were compared with those of cassava, potato, and corn starch. The particle size of C. esculentus starch was approximately 2 to 15 μm. The gelatinization temperature was 70.5 °C, and the peak viscosity was similar to cassava but with better thermal stability. Like other tuber starches, C. esculentus starch had higher swelling power and solubility at 85 °C.