A door operator of the fireproof door comprises a force applying end for releasing the reel of a door curtain, and a loading end for sustaining the weight of the door curtain. The rotary shaft comprises an internal (central) shaft and a plurality of external shaft coupled to each other via a clutch mechanism. The force applying end and the loading end act on the internal shaft and the external shaft respectively. A torsion spring brake mechanism is provided to resist the potential energy of the loading end by varying the inner diameter of the torsion spring so as to constantly restrain the rotary shaft from rotating, or to release the rotary shaft when the brake mechanism is subjected to an external force from the force applying end.