Edit /etc/apt/source.list
# Deb cdrom: [Ubuntu 4.16 LTS _Xenial Xerus_ - Release amd64 (20160420.1)] / great main restricted deb [arch = amd64, i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ great main restricted deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted deb [File = amd64, i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ great universe deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates universe deb [File = amd64, i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ great multiverse deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates multiverse deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security universe deb [arch=amd64,i386] http://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security multiverse