If your Ubuntu 18.04 is newly installed and you have not configured a download source for Ubuntu yet, you need to modify the download source first.
Back up the download source first

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

Then execute the instructions to edit the download source

sudo gedit /etc/apt/sources.list

Empty the contents of the file and paste the following into it

deb http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse

Then execute the instructions to update the download source

sudo apt-get update

software Ubuntu needs to be configured after success

Select Settings
Open the Software and Update dialog box and make sure that restricted, universe, and Multverse are ticked before you change Download From to domestic.
Then set Updates to ensure that all three options under install updates from are checked.
Modify the download source of ROS to China University of Science and Technology

	sudo sh -c '. /etc/lsb-release && echo "deb http://mirrors.ustc.edu.cn/ros/ubuntu/$DISTRIB_CODENAME main" > /etc/apt/sources.list.d/ros-latest.list'

Import key

sudo apt-key adv --keyserver 'hkp://pgp.mit.edu:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654

Modify houst

sudo vi /etc/hosts

Paste the following at the end of the file

199.232.4.133 raw.githubusercontent.com 

Start formal ROS installation after all preparations have been made
Perform the following instructions to install ROS

sudo apt update
sudo apt install ros-melodic-desktop-full

Initialize ros

sudo rosdep init
rosdep update

Solution to rosdep update time out problem
Execute instructions

sudo gedit /usr/lib/python2.7/dist-packages/rosdep2/sources_list.py

Modify the value of DOWNLOAD_TIMEOUT = 15.0 a little larger, and I set it to 500.
Then execute the instructions

sudo gedit /usr/lib/python2.7/dist-packages/rosdep2/rep3.py

Modify the value of DOWNLOAD_TIMEOUT = 15.0 a little larger, and I set it to 500.
Last Execute Directive

sudo gedit  /usr/lib/python2.7/dist-packages/rosdep2/gbpdistro_support.py

Modify the value of DOWNLOAD_TIMEOUT = 15.0 a little larger, and I set it to 500.
After completing the above, execute the instructions

rosdep update

Environment Settings

echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc

source ~/.bashrc

Building dependencies on packages

sudo apt install python-rosinstall python-rosinstall-generator python-wstool build-essential

Test for successful installation
Open Terminal-1 Execution Instruction

roscore

Open Terminal 2 Execution Instruction

rosrun turtlesim turtlesim_node

Open Terminal 3 Execution Instruction

rosrun turtlesim turtle_teleop_key