清华大学开元软件镜像站：https://mirrors.tuna.tsinghua.edu.cn/help/centos/

第一步执行，备份一下，没有可以不用备份
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak

第二步编辑文件，把下面的代码拷贝进去，保存退出
vim /etc/yum.repos.d/CentOS-Base.repo

# 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

baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/os/$basearch/

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

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates

[updates]

name=CentOS-$releasever - Updates

baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/updates/$basearch/

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

gpgcheck=1

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful

[extras]

name=CentOS-$releasever - Extras

baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/extras/$basearch/

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

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

baseurl=https://mirrors.tuna.tsinghua.edu.cn/centos/$releasever/centosplus/$basearch/

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

gpgcheck=1

enabled=0

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

最后清缓存sudo yum makecache