There are many sources of Ubuntu in China, including: NetEase source (this used before, fast), Ali, there are many sources of education network, such as: Tsinghua source, Zhong Ke source.
Here is the source list of Zhong Ke ubuntu16.04, which can be obtained here: https://lug.ustc.edu.cn/repogen/
To edit the/etc/apt/sources.list file, add the following entry at the top of the file (make the appropriate backup before you do the operation):
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
Then enter:
sudo apt-get update
Update the source.
Ubuntu 16.04 LTS Modify domestic source (take Zhong ke source for example)