The invention embodiment discloses a dual system and the dual on-line upgrading method. The dual system consists of an upgrading module, a switching module, a host machine and a standby machine. The dual on-line upgrading method includes: the upgrading for the host machine and the standby table corresponding to the main use table in the standby machine database; the upgrading for the application program of the standby machine; the switching for the application program and the database of the host machine and the standby machine; the switching for the main use table and the standby table in the standby machine; and the upgrading for the application program of the host machine. The dual system and the dual on-line upgrading method aim at stopping the outward service providing when switchingthe host machine and the standby machine as well as switching the main use table and the standby table in the standby machine, the stopping time for the outward service providing is reduced, and the influence for the service normal operation is reduced.