1. An offline download method adapted to be used with an offline download system comprising a cloud storage storing pre-downloaded files, the offline download method comprising steps of: receiving an offline download request for a requested file from a client, selecting at least one substitute file from the pre-downloaded files stored in the cloud storage when the requested file is not found m the cloud storage, wherein the at least one substitute file is determined to be similar to the requested file according to a similarity between the at least one substitute file and the requested file, sending first information comprising at least one filename of the at least one substitute file and at least one associated download link to the cloud storage, and the client downloading one of the at least one substitute file from the cloud storage by connecting to the at least one associated download link.