http://mirror.ubuntu9.com/index.php?t=s&r=precise该链接给出了很多的源，可以用ping找出对你来说最快的源
例如
cd /etc/apt/
cp source.list source.list.bak
wget http://mirror.ubuntu9.com/topmirror/sourceslist/all/a60c984d6752a3bb0c2bf12b53f432e7.list
mv a60c984d6752a3bb0c2bf12b53f432e7.list source.list
apt-get update