Multi-function printer with a locking device and operational method thereof

A multi-function printer is provided. A trigger piece is fixed to the first module and stretched into a second module. A wedging piece is disposed in the second module and protruded from the second module. An actuating piece and a join piece are disposed in the second module. The actuating piece has a first end connected to the actuating piece and a second end positioned close to the trigger piece. When the three modules are stacked together, the trigger piece goes against the wedging piece and the actuating piece goes against a third module. When the first module is rotated, the trigger piece is separated from the wedging piece such that the wedging piece can be wedge with the third module. When the second module is rotated, the actuating piece drives the joint piece to wedge with the trigger piece.