#deb cdrom:[linux Mint _sarah_-release AMD64 20160628]/xenial contrib main non-free
Deb Http://mirrors.ustc.edu.cn/ubuntu/xenial main restricted universe multiverse
Deb Http://mirrors.ustc.edu.cn/ubuntu/xenial-security main restricted universe multiverse
Deb Http://mirrors.ustc.edu.cn/ubuntu/xenial-updates main restricted universe multiverse
Deb http://mirrors.ustc.edu.cn/ubuntu/xenial-proposed main restricted universe multiverse
Deb Http://mirrors.ustc.edu.cn/ubuntu/xenial-backports main restricted universe multiverse
DEB-SRC http://mirrors.ustc.edu.cn/ubuntu/xenial main restricted universe multiverse
DEB-SRC http://mirrors.ustc.edu.cn/ubuntu/xenial-security main restricted universe multiverse
DEB-SRC http://mirrors.ustc.edu.cn/ubuntu/xenial-updates main restricted universe multiverse
DEB-SRC http://mirrors.ustc.edu.cn/ubuntu/xenial-proposed main restricted universe multiverse
DEB-SRC http://mirrors.ustc.edu.cn/ubuntu/xenial-backports main restricted universe multiverse
Use sudo gedit/etc/apt/source.list to find the file and replace it with the above content
Finally sudo apt-get update can