A composite pull-chain disc mechanism with brake function for a door operator comprises a torsional spring having de-twisting and twisting sides. The weight of a door curtain always acts on the de-twisting sides, so that the inner diameter of the torsional spring is induced to shrink to brake the rotation of the central shaft of the door operator. If twisting sides bears an external force, the inner diameter of the torsional spring is induced to enlarge to release braking of the central shaft. Furthermore, the composite pull-chain disc mechanism can be adapted to an electric door operator. In the case of detaching an electric motor mechanism during maintenance, the door operator can still be operated to open/close a door in a manual mode through the composite pull-chain disc mechanism.