\usepackage[a4paper]{geometry}
\usepackage{graphicx}
\usepackage{eso-pic}

\newlength{\positionbarre}
\setlength{\positionbarre}{2.5cm}% à changer selon les besoin
\AddToShipoutPicture{%                      \/
\put(\LenToUnit{\positionbarre},\LenToUnit{0.05\paperheight})
{\begin{picture}(0,0)(0,0) \line(0,1)
{\LenToUnit{0.85\paperheight}}\end{picture}}
}%            /\
\makeatother