1、备份

cd /etc/yum.repos.d/
mv  CentOS-Base.repo  CentOS-Base.backup

2、下载新的CentOS-Base.repo 到/etc/yum.repos.d/

wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo

如果出现解析错误
Could not resolve host: mirrors.aliyun.com;
解决：添加 DNS 解析即可

vi resolv.conf 

在里面添加域名解析服务

nameserver 8.8.8.8
nameserver 114.114.114.114

重新拉取试下

3、之后运行yum clean all 清除缓存，运行 yum makecache 生成新的缓存

 yum clean allyum makecache