Parallel four-bar mechanism

A parallel four-bar mechanism is disclosed, having the second and third bars being parallel to each other and having their opposite ends hinged to the first and fourth bars respectively. A stopping block is provided with a stopping slot, and a movable stopping pin is configured to be engageable with the stopping slot. When an actuating member drives the stopping pin to disengage from the stopping slot, the second and third bars are able to translation relative to each other; and when the actuating member drives the stopping pin into the stopping slot, the second and third bars are locked with each other, with the second and third bars being not able to translation relative to each other, and thus the whole parallel four-bar mechanism is locked in a preset angular position.