1、修改配置

vim /boot/grub2/grub.cfg

2、定位到如下这段

if [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=5
fi

3、修改set timeout 为1，为0无效，只需要改第一个timeout。

if [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=1

# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=5
fi