A screw locking device includes a feeding mechanism, a conveying mechanism, and a pneumatic screw driver. The conveying mechanism includes a base, a pneumatic driver slidably mounted on the base, a pair of blocking members slidably mounted on the base, a conveying bridge, a flexible tube, and a controller. Each blocking member includes a blocking end positioned below the pneumatic driver. The conveying bridge communicates with the feeding mechanism, and extends to the blocking ends. The flexible tube aligned with a gap between the blocking ends, and communicates with the pneumatic screw driver.