Abstract: To solve the problems of slow focusing in a stepper motor lens, caused by a stepper motor used for low-frequency driving, and the lack of focus, caused by an out-of-step stepper motor used for high-frequency driving, the open-loop control algorithm of a stepper motor for an STM lens is designed by analyzing the motor's acceleration and deceleration action and its acceleration-deceleration curves during the focusing process. Based on the constant speed of the motor during the unit control period, the algorithm transforms the position control problem of the stepper motor into a pulse output problem in the unit control cycle, which realizes a skillful combination of speed control and position control. The control circuits are built for the STM lens focusing motor. The experimental results show that this algorithm can improve the open-loop control precision of the stepper motor, effectively suppressing the out-of-focus problem caused by overshoot and out-of-step stepper motors.