\documentclass{article}
\usepackage{tabularx} % 需导入宏包
\usepackage{amsmath}
\usepackage{booktabs}
\begin{document}
\begin{table}[!htbp]
\centering
\begin{tabularx}{\textwidth}{Xcc} % X为自动伸缩列,c为固定宽度列
\toprule[1.5pt]
符号 & 说明 & 单位\\
\midrule[1pt]
5 & 269.8 & 0.04089\\
10 & 421.0 & 0.04089\\
20 & 640.2 & 0.04089\\
\bottomrule[1.5pt]
\end{tabularx}
\label{tab:001}
\end{table}
\end{document}
我也是在lshort-zh-cn
中找到你这个case里的合适方法的
\documentclass[12pt]{ctexart}
% \usepackage[showframe]{geometry}
\usepackage{tabularx}
\usepackage{array}
\usepackage{booktabs}
\begin{document}
\begin{table}[!htbp]
\centering
\begin{tabularx}{\textwidth}{*{3}{>{\centering\arraybackslash}X}}
\toprule[1.5pt]
符号 & 说明 & 单位\\
\midrule[1pt]
5 & 269.8 & 0.04089\\
10 & 421.0 & 0.04089\\
20 & 640.2 & 0.04089\\
\bottomrule[1.5pt]
\end{tabularx}
\label{tab:001}
\end{table}
\end{document}