A connecting device includes a base, a locking member, linkage, and a detector. The locking member is rotatably disposed on the base and configured to rotate about an axis to a locked position along a first rotational direction and to an unlocked position along a second rotational direction opposite to the first rotational direction. The locking member has an engaging portion at a peripheral edge thereof. The linkage is rotatably disposed on the base and includes an abutting portion configured to abut against the peripheral edge. When the locking member is rotated to the locked position, the abutting portion is moved to and is engaged with the engaging portion accompanied with a rotation of the linkage. The detector is disposed on the base and configured to detect the rotation of the linkage.