The invention is a off-line preserve mode of network game virtual property, which launches at the request of virtual property preserved by the client computer games, game server host test the legality of the request, if legitimate, generates a unique number for the virtual property, and the data frame of the virtual property after using key encryption, sent to the client computer games. The games client computer will be on behalf of encrypted data frame of the virtual property in the form of a document preservation to mobile storage equipment, notice game server host after success, then the state of the virtual property is set to ''off-line preserved.'' Since the game player, in another game client computer, can re-activate the virtual property by using documents of virtual property kept frontly. This design can improve the safety of virtual property, and realize the virtual property in kind.