第一步：

sudo sed -e 's|^mirrorlist=|#mirrorlist=|g'
-e 's|^#baseurl=http://mirror.centos.org/centos|baseurl=https://mirrors.ustc.edu.cn/centos|g'
-i.bak
/etc/yum.repos.d/CentOS-Base.repo

第二步：

vi /etc/yum.repos.d/CentOS-Base.repo

插入以下语句：

CentOS-Base.repo

The mirror system uses the connecting IP address of the client and the

update status of each mirror to pick mirrors that are updated to and

geographically close to the client. You should use this for CentOS updates

unless you are manually picking other mirrors.

If the mirrorlist= does not work for you, as a fall back you can try the

remarked out baseurl= line instead.

[base]
name=CentOS-$releasever - Base

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra

baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

released updates

[updates]
name=CentOS-$releasever - Updates

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra

baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

additional packages that may be useful

[extras]
name=CentOS-$releasever - Extras

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra

baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

additional packages that extend functionality of existing packages

[centosplus]
name=CentOS-$releasever - Plus

mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra

baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

第三步：yum clean all
yum makecache