The utility model discloses a speed regulating circuit. After a switch is switched off according to the control of an outer device, a Hall sensor is used for realizing the regulation control on the speed of an electric bicycle. If the Hall sensor is out of control, the outer device can also be used for controlling the switch to be switched off, and further, a transmission path of voltage signals from a power supply to the motor controller is cut, so the moving of the electric bicycle is stopped, the reliability of the speed regulation control is improved, and meanwhile, the occurrence of safety accidents is avoided.