一种自动式螺丝起子，包含：一本体，中空柱状体且其具有一开孔；一开关单元，设置于该开孔中且具有一用以切换一锁紧模式及一卸下模式的球状部；一滑动限位单元，设置于该本体内，其包括一第一滑轨与一第二滑轨；一螺丝套单元，与该滑动限位单元相连接，且具有一螺丝头；当该开关单元切换为该锁紧模式时，该开关单元控制该球状部于该滑动限位单元之第一滑轨上滑动，且连动该螺丝头以一第一方向旋转。

This invention provides an automatic screw driver. It comprises a body with a hollow cylindrical body and having an opening hole, a switching unit with disposed in the opening hole and having a spherical portion to switch between a locking mode and a remove mode, a sliding stopper unit with disposed in the body and comprising a first slide rail and a second slide rail; a screw cap unit with connecting to the sliding stopper unit and having a screw head. When the switching unit switches to the locking mode, the switching unit controls the spherical portion to slide on the first slide rail of the sliding stopper unit and it connects to the screw head to rotate in a first direction.