The common process of installing double operating systems is install windows 7 first and then install ubuntu aside.
In the process, I encountered some problems, and I hope people will avoid this through my writing.
When installing windows 7 by a U pan, it's extremely easy to partition the disk through the user interface.
Actually, it's not right. Because the partition will be set as primary partition by default, which will hold you back when you try to install another system aside.
The right thing is you press "SHIFT + F10" to the command window, use "diskpart" tool, to create, delete, clean the disk.
Through that tool, you can easily create primary,logical,extended partitions with some commands. It would be better if you just create only one primary partition and leave the other to be an extended partition.
And you can then edit the extended partition by partitioning.
Then you wouldn't encounter the problem I had met before when installing ubuntu.