A safety fence includes a door unit, a locking unit mounted on the door unit, a switching mechanism mounted on the door unit and connected with the locking unit, and a positioning and restoring unit mounted on the door unit. The door unit includes a first rail, a second rail and a door. Thus, when the door unit is not opened to reach the predetermined angle, the door is driven by the positioning and restoring unit and is returned and moved backward relative to the first rail to abut the second rail so that the door is locked onto the second rail by the locking unit so as to close the door unit automatically.