You can use caption package together with setspace package. This is what caption documentation says:

Further, you can use setspace package to adjust spacing for the whole document by using the commands it provides as noted by egreg.
A MWE will be:
\documentclass{book}
\usepackage{graphicx,kantlipsum,setspace}
\usepackage{caption}
\captionsetup[table]{font={stretch=1.2}} %% change 1.2 as you like
\captionsetup[figure]{font={stretch=1.2}} %% change 1.2 as you like
%% or
%% \captionsetup{font={stretch=1.2}} %% this affects both figure and table
%\renewcommand{\baselinestretch}{1.5}
\setstretch{1.5}
\begin{document}
\chapter{Some chapter}
\kant[1-3]
\begin{table}[htb]
\centering
\caption{\kant[1]}\label{tab:mytable}
Some table comes here
\end{table}
\kant[4-8]
\begin{figure}[htb]
\centering
\includegraphics[width=4cm]{example-image-a}
\caption{\kant[2]}\label{fig:myfig}
\end{figure}
\end{document}

setspacepackage that already takes care of that. – egreg Jan 10 '14 at 11:55