The invention discloses a composite vacuum on-load tap-changer with an external selector. The composite vacuum on-load tap-changer comprises a switch oil chamber and a switch body which inserts into the switch oil chamber. The switch oil chamber comprises an insulation tube, a tap-changer installation flange fixed on a head of the insulation tube, a supporting flange and a tube bottom installed on the bottom of the insulation tube. The composite vacuum on-load tap-changer is characterized in that: the selector is installed on an outer side of the switch oil chamber insulation tube through an upper supporting arm which is connected into one body with the supporting flange and a lower supporting arm which is connected into one body with the tube bottom; the selector is driven to work through a selector driving mechanism installed on the supporting flange. According to the invention, because of the external selector, it is possible to design the switch body and the switch oil chamber into a plug-in type.