I read the wiki "https://github.com/Valloric/YouCompleteMe/wiki/Building-Vim-from-source".
But it misleads me.
Please DO NOT configure vim like this on Ubuntu 14.04.5 LTS
--enable-pythoninterp=yes
--with-python-config-dir=/usr/lib/python2.7/config
--enable-python3interp=yes
--with-python3-config-dir=/usr/lib/python3.5/config
Make sure only select one from python2 or python3.