A method and device for upgrading a guest operating system (OS) of an active Virtual Machine (VM). The method includes: replicating the active VM to generate a backup VM; upgrading a guest OS of the backup VM; merging changes to be made to a virtual disk image file during running of the active VM and changes to be made to the virtual disk image file during the upgrading; pausing the backup VM; migrating an active process of a Java application program running in the active VM into the backup VM to generate a backup process; migrating resources occupied by the active process of the Java application program into the backup VM such that the backup process occupies the same resources; and resuming running of the backup VM. The device includes: a replicating unit; an upgrading unit; a merging unit; a migrating unit; and a control unit.