An obstacle avoidance system based on sonar for an unmanned aerial vehicle includes an electric motor, a single sonar module, and a control panel. The sonar module rotates with the electric motor. The control panel controls a rotation of the electric motor, thereby controlling the direction in which obstacles to the flight of the unmanned aerial vehicle are detected by the sonar module. The unmanned aerial vehicle includes a fuselage, the sonar obstacle avoidance system, and an aerial vehicle controller. A sonar obstacle avoidance method is also disclosed.