找到答案了,@u70550 谢谢大佬帮忙
\documentclass{ctexbeamer}
\usepackage{tikz}
\usetikzlibrary{calc}
\setbeamercolor{normal text}{bg=teal}%背景色
\setbeamertemplate{footline}{
\begin{tikzpicture}
\def\r{1pt}
\def\d{0.3pt}
\coordinate (A) at (0,1em);
\coordinate (B) at ($(A)+(0:{\paperwidth})$);
\draw[red,line cap=rect] (A)--(B);
\node[anchor=south west,inner sep=0pt,outer sep=0pt,fill=red,draw=red] at (0,0){\color{white}测试测试测试};
\end{tikzpicture}
}
\begin{document}
\begin{frame}
测试
\end{frame}
\end{document}

问 footline用tikz画直线为什么最左边有空隙