\documentclass{book}
\usepackage{xepersian}
\makeatletter
\def\@chapter[#1]#2{\ifnum \c@secnumdepth >\m@ne
                       \if@mainmatter
                         \refstepcounter{chapter}%
                         \typeout{\@chapapp\space\thechapter.}%
                         \addcontentsline{toc}{chapter}%
                                   {\chaptername~\protect\numberline{\@tartibi\c@chapter:}~~#1}%
                       \else
                         \addcontentsline{toc}{chapter}{#1}%
                       \fi
                    \else
                      \addcontentsline{toc}{chapter}{#1}%
                    \fi
                    \chaptermark{#1}%
                    \addtocontents{lof}{\protect\addvspace{10\p@}}%
                    \addtocontents{lot}{\protect\addvspace{10\p@}}%
                    \if@twocolumn
                      \@topnewpage[\@makechapterhead{#2}]%
                    \else
                      \@makechapterhead{#2}%
                      \@afterheading
                    \fi}
%%%%%%%%%%%%%%%%%%%%%%%%%%
\renewcommand*\l@section{\@dottedtocline{1}{4.3em}{2.3em}}        
\renewcommand*\l@subsection{\@dottedtocline{2}{6.5em}{3.2em}}  
\makeatother
\begin{document}
\tableofcontents
\chapter{مقدمه}
\section{بخش}
\subsection{زیربخش}
\chapter{پیش‌گفتار}

مقداری متن ...
\end{document}