A powerless helical locking mechanism for a door includes a screw with a variable lead angle connected with a power source, and a self-adaptive nut connected to the door. The helical slot of the screw is divided into a working segment with the helical lead angle greater than the friction angle, a closing segment with the helical lead angle smaller than the friction angle, and a transition segment between the closing and working segments. The power source actuates the screw to rotate bidirectionally.