Finally, thanks to a lot of help from Derrick Sobodash, a user of the Beijing GNU/Linux user group I managed to install Debian into the Lemote Yeeloong.
0. Download the debian loongson image: http://bjlx.org.cn/loongson/loongson2_debian5_20090108.tar.bz2
1. Use gparted to format a partition on a USB disk to ext3.
2. Mount it, change to the folder inside the usb /, and run `tar xjpvf /yourpath/loongson2_debian5_20090108.tar.bz2
3. Open /media/YOUR_NEW_EXT3/etc/fstab and comment out all lines except proc.
4. Run `sync`. Be patient. Remove your USB disk.
5. Start the Yeelong. Open terminal. Remount / as rw instead of ro this way: mount -o remount, rw /
6. Edit /boot/boot.cfg. Make sure it has “showmenu 1″. Add these lines.
title Debian on Yeeloong
kernel /dev/fs/ext2@usb0a/boot/vmlinux-yeeloong args console=tty root=/dev/sda1 no_auto_cmd rootdelay=10
7. Power down. Plug in your USB key.
8. Press DEL. At the pmon prompt write: load /dev/fs/ext2@usb0a/boot/vmlinux-yeeloong g console=tty root=/dev/sda1 no_auto_cmd rootdelay=10
Then the system will boot into your new shining debian. 
Derrick Sobodash suggested:
if you get a kernel panic about not being able to mount root (/), check if it is mapping it as sda. The supplied kernel has a quirk where it mounts and unmounts the device creating it as sdb the second time. My new kernel for medan doesn’t have this problem. If you continue to have trouble, I suggest trying a USB hard disk, not a USB stick. I’ve had a lot of bad experiences with sticks flaking out and writing 0s inside of files. A USB hard disk will not have this problem.
Thanks also to all the #beijinglug friends on irc.freenode.net for all your help and support.
If you found this guide helpful, please consider joining the Beijing GNU/Linux user group and help them. They really deserve it.
