The ctexart sets the indent after the section title in \@startsection. There should be a key to change the behaviour, but sadly I don't understand Chinese, and the documentation seems to be in Chinese. The key should be anything related to the word afterindent, if you understand the documentation search for that.
Therefore the following hacks its way through ctexart:
\documentclass[UTF8]{ctexart}
\usepackage{xpatch}
\makeatletter
\xpatchcmd\@startsection{\@afterindenttrue}{\@afterindentfalse}{}
{%
\GenericError{}{Patching \noexpand\@startsection failed}{}
{Read the manual of 'ctexart' and search for the key 'afterindent'.}%
}
\makeatother
\begin{document}
\section{A}
\subsection{A-A}
before text
\begin{itemize}
\item a
\item b
\end{itemize}
after text
\end{document}
\noindent before textbut this is not the behaviour of the standard classes. Please provide a full minimal working example which reproduces the issue, possibly starting with\documentclass{...}and ending with\end{document}. – campa Jun 20 '18 at 07:46\noindentor\setlength\parindent{0pt}in the preamble help? – Bobyandbob Jun 20 '18 at 07:47articleclass.. which class are you using?? Add a mwe. – David Jun 20 '18 at 07:53ctexart. I found it also works. but they are left aligned with section number. It works if I useflushleft. but how if there anyway I can add the indent? – michael Jun 20 '18 at 08:02