首先是MWE，其输出如下：
\documentclass{book}
\usepackage{titlesec}%
\let\oldtitleline\titleline
\renewcommand{\titleline}{\oldtitleline*}
\setlength{\titlewidth}{0.3\textwidth}%
\newcommand{\periodafter}[1]{#1.}%
\titleformat{\chapter}{\Large\bfseries\centering}{\chaptertitlename\ \thechapter.}{\wordsep}{\periodafter}[\addvspace{-0.1\baselineskip}{\hfill\titlerule[0.4pt]\hfill\null}]
\makeatother
\setlength{\titlewidth}{0.15\textwidth}%
\titleformat{\section}{\centering}{\thesection.}{\wordsep}{\periodafter}[\addvspace{-0.1\baselineskip}{\hfill\titlerule[0.4pt]\hfill\null}]
\begin{document}
\chapter{A chapter title}
\section{A section title}
\end{document}
如您所见，我重置了 的值来\titlewidth更改章节标题的标题规则长度，但不起作用？那么有没有办法修复它，即让章节标题具有不同长度的标题规则？
答案1
使用您的代码，\titleline是绝不叫。
\documentclass{book}
\usepackage{titlesec}
\newcommand{\periodafter}[1]{#1.}
\titleformat{\chapter}
{\Large\bfseries\filcenter}
{\chaptertitlename\ \thechapter.}
{\wordsep}
{\periodafter}
[%
\addvspace{-0.1\baselineskip}%
\hspace*{\fill}\rule{0.3\textwidth}{0.4pt}\hspace*{\fill}%
]
\titleformat{\section}
{\filcenter}
{\thesection.}
{\wordsep}
{\periodafter}
[%
\addvspace{-0.1\baselineskip}%
\hspace*{\fill}\rule{0.15\textwidth}{0.4pt}\hspace*{\fill}%
]
\begin{document}
\chapter{A chapter title}
\section{A section title}
\end{document}
我建议对 做出不同的定义\periodafter，请看示例以了解原因。
\documentclass{book}
\usepackage{amsthm} % for \@addpunct
\usepackage{titlesec}
\makeatletter
\newcommand{\periodafter}[1]{#1\@addpunct{.}}
\makeatother
\titleformat{\chapter}
{\Large\bfseries\filcenter}
{\chaptertitlename\ \thechapter.}
{\wordsep}
{\periodafter}
[%
\addvspace{-0.1\baselineskip}%
\hspace*{\fill}\rule{0.3\textwidth}{0.4pt}\hspace*{\fill}%
]
\titleformat{\section}
{\filcenter}
{\thesection.}
{\wordsep}
{\periodafter}
[%
\addvspace{-0.1\baselineskip}%
\hspace*{\fill}\rule{0.15\textwidth}{0.4pt}\hspace*{\fill}%
]
\begin{document}
\chapter{A chapter title}
\section{A section title!}
\end{document}
