A door operator of a fireproof door comprises a force applying end through which a driving force is transmitted by an input shaft; an output end for sustaining the weight of the door curtain which is transmitted to a central shaft through an output shaft; a clutch mechanism connecting the central shaft to the input shaft; a brake mechanism including a torsion spring disposed on the input shaft which bears the loading force transferred through the central shaft to vary the inner diameter of the torsion spring so as to restrain the input shaft from rotating; when an external force is applied to the input end, the inner diameter of the torsion spring is extended so that the input shaft is rotatable.