The open-loop control of the stepper motor is simple, but it has low positioning accuracy, large torque pulsation, and low energy efficiency. While the sensorless closed-loop field-oriented control has high precision, small torque pulsation, and high energy efficiency, it has problems such as complex structure and a large calculation. To achieve a speed closed-loop, a sensorless control method combining an improved sliding mode back electromotive force observer is proposed, combining a high-order super-twisting sliding mode observer (SMO) and a third-order phase-locked loop (PLL). Super-twisting SMO improves the chattering problem of traditional one-order SMO, and third-order PLL can eliminate steady-state errors during acceleration and deceleration processes, enhancing the estimation accuracy and stability of the rotor position and speed. Finally, an experimental platform on the basis of a two-phase hybrid stepper motor was built to verify the effectiveness and reliability of the control strategy.