\begin{tikzpicture}

\node [anchor=west] (eq1) at (0,0) {$\hat{\theta}$};
\node [anchor=west] (eq2) at ([yshift=-0.2em]eq1.east) {=};
\node [anchor=west,inner sep=2pt] (eq3) at ([yshift=-0.0em]eq2.east) {$\argmax$};
\node [anchor=north,inner sep=1pt] (eq3part2) at ([yshift=-0.2em]eq3.south) {\scriptsize{$\theta$}};
\node [anchor=west,inner sep=2pt] (eq4) at ([xshift=0.1em]eq3.east) {$\textrm{P}_{\theta}(s|t)$};

{
\node [anchor=west,inner sep=2pt,fill=red!20,minimum height=1.35em] (eq3) at ([yshift=-0.0em]eq2.east) {$\argmax$};
\node [anchor=west,inner sep=2pt,fill=green!20] (eq4) at ([xshift=0.1em]eq3.east) {$\textrm{P}_{\theta}(s|t)$};

\node [anchor=north,draw,inner sep=3pt,fill=red!20] (eq3label) at ([yshift=-1.5em]eq3.south west) {\footnotesize{\textbf{求最优参数}}};
\node [anchor=north,draw,inner sep=3pt,fill=green!20] (eq4label) at ([yshift=-1.5em]eq4.south east) {\footnotesize{\textbf{目标函数}}};

\draw [->,thick] ([xshift=-1em]eq3.south) .. controls +(south:1em) and +(north:1em) .. (eq3label.north);
\draw [->,thick] (eq4.south) .. controls +(south:1em) and +(north:1em) .. (eq4label.north);
}

\end{tikzpicture}