The utility model discloses a device for on-site replacement of a rotor vapor seal. The device comprises a bottom plate, two steel strips, a transverse cutter rest and a small cutter rest, wherein the bottom plate is used for being connected with the rotor vapor seal, the two steel strips are fixedly arranged at the two ends of the bottom plate, the transverse cutter rest is arranged on the two steel strips, and the small cutter rest is arranged on above the transverse cutter rest. The transverse cutter rest comprises a dovetail guide rail and a guide groove matched with the dove tail guide rail, long grooves with upper openings are formed in the steel strips in the length direction, the sections of the grooves are in the shape of the Chinese character 'tu', the bottom of the dovetail guide rail is arranged and matched with the long grooves in the shape of the Chinese character 'tu' by aid of bolts, the smaller cutter rest is fixedly arranged on the top face of the guide groove, and the guide rail direction of the small cutter rest is perpendicular to the direction of the dovetail guide rail. The device for on-site replacement of the rotor vapor seal has the advantages that a cutting device is arranged on a vapor seal cylinder body, the transverse cutter rest and the small cutter rest are arranged on the cutting device, transverse and longitudinal cutter feeding in a matching mode is achieved, multidirectional cutting is conducted, the vapor seal does not need to be disassembled and turned by other large lathes, the vapor seal is turned on site, the procedures of disassembling and hoisting are omitted, efficiency is improved, and labor strength is reduced.