vi /etc/pam.d/su

#%PAM-1.0
auth sufficient pam_rootok.so

Uncomment the following line to implicitly trust users in the “wheel” group.

#auth sufficient pam_wheel.so trust use_uid

Uncomment the following line to require a user to be in the “wheel” group.

#auth required pam_wheel.so use_uid
auth include system-auth
account sufficient pam_succeed_if.so uid = 0 use_uid quiet
account include system-auth
password include system-auth
session include system-auth
session optional pam_xauth.so

第四行去掉注释

然后在外部执行

usermod -G wheel 你自己的用户。以后就直接root了。
su - root
不用密码