centos7：http://mirrors.163.com/centos/7/os/x86_64/Packages/
centos6:http://mirrors.163.com/centos/6/os/i386/Packages/
找到以下三个包：
yum-3.4.3-158.el7.centos.noarch.rpm
yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
yum-plugin-fastestmirror-1.1.31-45.el7.noarch.rpm
因为我是centos7所以 找的是这三个包，
然后使用wget下载这三个包
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-3.4.3-158.el7.centos.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-45.el7.noarch.rpm
再用：rpm -ivh yum-* 安装yum
OK