Using an existing Wubi will download all the files successfully, however the OS will be unable to install because of bad files. Is the mirror site of Ubuntu installation files GFWed in China?
If you are in China and you tried to download Ubuntu from the Main Servers, you may get a corrupted copy because the servers are usually located in the US or the UK.
It is possible that the GFW can block the servers, though. So I would suggest you to download from the cn99 mirror in China and do a full install with the official Ubuntu Image.
32-bit CD: ubuntu-12.04-desktop-i386.iso
64-bit CD: ubuntu-12.04-desktop-amd64.iso
You have a few options.
Try Installing From a Full Ubuntu