\documentclass[11pt,a4paper,twocolumn]{article}
\usepackage{multicol}
\usepackage{blindtext}
\begin{document}

\blindtext

\onecolumn
\begin{figure}[!ht]
\centering 
\rule{0.9\linewidth}{0.2\linewidth}
\caption{this is caption of figure}
\end{figure}


\twocolumn

\blindtext
%\blinddocument

\end{document}