更新源文件

sudo gedit /etc/apt/sources.list

#北京理工大学 deb http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe deb http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-backports main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-proposed main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-security main multiverse restricted universe deb-src http://mirror.bjtu.edu.cn/ubuntu/ precise-updates main multiverse restricted universe

sudo apt-get update

将文件内容全部替换为北理工镜像即可解决