% 另外，我怎样才能减少目录中前言之间的间距。
\documentclass[a4paper, 12pt]{report}
\usepackage{fancyhdr}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\huge\bfseries}{\chaptertitlename\ \thechapter}{20pt}{\Huge}
\titlespacing*{\chapter}{0pt}{-20pt}{20pt}
\pagenumbering{roman}
\begin{document}
\chapter*{\underline{CANDIDATE'S DECLARATION}}%
\addcontentsline{toc}{chapter}{\numberline{}Candidate's Declaration}%
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
xxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
xxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
\par\noindent\rule{\textwidth}{0.4pt}
\chapter*{\underline{CERTIFICATE}}%
\addcontentsline{toc}{chapter}{\numberline{}Certificate}%
This is to certify that the above statement made by the candidate is correct to best of my knowledge and belief.
\\
\\
\\
\\
\\
\begin{flushright}
xxxxxxxxxxxxxxx\\
Professor\\
xxxxxxxxxxxxxx\\
xxxxxxxxxxx
\end{flushright}
\tableofcontents
\pagenumbering{arabic}
\chapter{Introduction}
\end{document}
并在中心
答案1
好吧，您可以重新定义\cleardoublepage，并且\clearpage什么也不做，这样您的第二个就不会有新页面\chapter*{}。要使两个章节都居中，您必须添加\centering并\titleformat在不再需要居中之后重新定义它...
请参阅以下代码（肮脏的黑客！）：
\documentclass[a4paper, 12pt]{report}
\usepackage{fancyhdr}
\usepackage{titlesec}
\usepackage{showframe}% <============ to visualize typing area and margins
\titleformat{\chapter}[display]
{\normalfont\huge\bfseries\centering}{\chaptertitlename\ \thechapter}{20pt}{\Huge}
% ^^^^^^^^^^ <================================
\titlespacing*{\chapter}{0pt}{-20pt}{20pt}
\pagenumbering{roman}
\begin{document}
\chapter*{\underline{CANDIDATE'S DECLARATION}}%
\addcontentsline{toc}{chapter}{\numberline{}Candidate's Declaration}%
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
xxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
xxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxxxxx\\xxxxxxxxxxxxxxxxxxxxxxxx\\
\par\noindent\rule{\textwidth}{0.4pt}\vspace{\baselineskip} % <=====
{\let\cleardoublepage\relax % <=====================================
\let\clearpage\relax % <============================================
\chapter*{\underline{CERTIFICATE}}%
\addcontentsline{toc}{chapter}{\numberline{}Certificate}%
}
This is to certify that the above statement made by the candidate is correct to best of my knowledge and belief.
\vspace{2cm} % <========================================================
\begin{flushright}
xxxxxxxxxxxxxxx\\
Professor\\
xxxxxxxxxxxxxx\\
xxxxxxxxxxx
\end{flushright}
\titleformat{\chapter}[display] % <=====================================
{\normalfont\huge\bfseries}{\chaptertitlename\ \thechapter}{20pt}{\Huge}
\tableofcontents
\pagenumbering{arabic}
\chapter{Introduction}
\end{document}
和结果（请注意，您不应该使用多个\\来获得垂直空间，\vspace{2cm}而是使用并根据需要更改值）：
正如您所看到的（包showframe显示 pdf 中的打字区域，删除/注释它以便实际打印）您的第一章有点太长，Certificate居中，Contents不居中。