China DOS Union

1: Where can I download it?
2: How do I use it, and under what system, Win98 or DOS?
3: What do I use to edit the source program?
4: What do I use to compile?
5: How do I link?
6: Is there a function manual?
7: Does it support Chinese?

Of course DJGPP and RHIDE are used under DOS, including in a DOS window.

There are downloads at http://www.simtel.net/pub/djgpp/.
For the specific method, please see its usage help. There are also plenty of function manuals online, but most of them are in English.

Too general! Too vague!
For example, BC has an IDE, you can create a project, compile, link, build
In BC's root directory there is bc.exe to open the IDE
What about DJGPP? RHIDE?
Which executable file is the IDE environment?
There are also plenty of function manuals online, so where's the URL?
An answer like this would be better left unanswered

All right, first download and extract DJGPP and RHIDE, then set the DJDIR environment variable (for example: SET DJDIR=C:\DJGPP), and then directly run RHIDE.EXE in the BIN folder to enter the IDE environment.

As for the various functions of DJGPP, you might as well look here:
http://www.simtel.net/pub/djgpp/