根指数你是指的3次根号x吗？那就应该是sqrt[3]{x}

\documentclass{ctexbook}
\usepackage{titlesec}
\titleformat{\chapter}{\centering\Huge\bfseries}{第\,\Roman{chapter}\,章}{1em}{}
\titleformat{\section}{\centering\Huge\bfseries}{\arabic{chapter}-\arabic{section}}{1em}{}
\begin{document}
\chapter{A}
\section{B}
a
\end{document}