vim /etc/apt/sources.list
然后用G跳转到最后一行，然后[ESC]切换到命令行模式，然后键入[o](表示在当前行后插入)。也可以多插入几个空行。这样可以有条理的和系统默认的区分。将来方便恢复，修改。
内容如下：
deb http://debian.ustc.edu.cn/ubuntu/ natty main multiverse restricted universe deb http://debian.ustc.edu.cn/ubuntu/ natty-backports main multiverse restricted universe deb http://debian.ustc.edu.cn/ubuntu/ natty-proposed main multiverse restricted universe deb http://debian.ustc.edu.cn/ubuntu/ natty-security main multiverse restricted universe deb http://debian.ustc.edu.cn/ubuntu/ natty-updates main multiverse restricted universe deb-src http://debian.ustc.edu.cn/ubuntu/ natty main multiverse restricted universe deb-src http://debian.ustc.edu.cn/ubuntu/ natty-backports main multiverse restricted universe deb-src http://debian.ustc.edu.cn/ubuntu/ natty-proposed main multiverse restricted universe deb-src http://debian.ustc.edu.cn/ubuntu/ natty-security main multiverse restricted universe deb-src http://debian.ustc.edu.cn/ubuntu/ natty-updates main multiverse restricted universe
然后我们可以：
sudo apt-get update
原文：http://www.cnblogs.com/magic-zero/p/6307642.html