Now I have solved the issue.
The mechanism is in a reserved direction. To open the door, I need to push towards the latchbolt direction instead of the hinge side. Guess my lock is bit of different from others.
Thank you guys for your helpful advice.
Harry, slide a slotted screwdriver into that 1/2 circle opening and push towards the hinge side of the door and it should open, if it's not broken. If the opening was square you would slide the flat screwdriver into the opening and turn it. That will open it.