\documentclass{article}
\usepackage{xepersian}
\settextfont{Yas}
\makeatletter
\long\def\@makecaption#1#2{%
  \vskip\abovecaptionskip
  \sbox\@tempboxa{#1: #2}%
  \ifdim \wd\@tempboxa >\hsize
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\makeatother
\begin{document}
\begin{figure}[h]
\caption{توضیح در سمت راست قرار می‌گیرد.}
\end{figure}
\end{document}