已在xelatex下验证成功

%Draft水印效果
\usepackage{eso-pic}
\usepackage{datetime}
\usepackage{tikz}
\usetikzlibrary{calc}
\newdateformat{mydate}{\THEYEAR-\twodigit{\THEMONTH}-\twodigit{\THEDAY}}
\newtimeformat{mytime}{\twodigit{\THEHOUR}:\twodigit{\THEMINUTE}}
\settimeformat{mytime}
\AddToShipoutPicture{%
\parbox[b][\paperheight]{\paperwidth}{%
\vfill
\centering%
\begin{tikzpicture}[remember picture,overlay]
\node [rotate=45,scale=10] at ($(current page.center) +(-1cm,1cm)$)
{\textcolor[gray]{0.8}{DRAFT}};
\node [rotate=45,scale=3] at ($(current page.center) +(1cm,-1cm)$)
{\textcolor[gray]{0.75}{编译时间：\mydate\today\hspace{10pt} \currenttime}};
\end{tikzpicture}%
\vfill
}}

来源：水木社区tex版面

另外有种加水印的方法但在xelatex下无效

\RequirePackage{draftcopy}
\draftcopyName{Copyright 2006 by xxx}{50}
\draftcopySetGrey{0.8}  
\draftcopyPageTransform{55 rotate}
\draftcopyPageX{80}  
\draftcopyPageY{-25}