\documentclass{article}
\usepackage[justification=centering]{caption}
\begin{document}
\begin{figure}
\centering
\fbox{here a graphic}
\caption{A long a long A long a long A long a long A long a long A long a long A long a long}
\end{figure}
\begin{figure}
\centering
\fbox{here a graphic}
\caption{A llong a long}
\end{figure}
\end{document}
cf example suivant si on ne charge pas le package