Q:

How to make a table just the first row centering?

Sorry for bothering you, but I try to make a table like this:
the first row centering and others can left or right aligned.
and my code:
\begin{table*}[!htbp]
\caption{Speech content guidance for SoHRI.}\label{tab:6}
\centering
\begin{tabular}
{| m{4.7cm} | >{\centering}m{1.2cm} | m{1.2cm}<{\centering}| m{1.4cm} <{\centering}| m{1.4cm} <{\centering}| m{1.4cm}<{\centering}|m{1.4cm}<{\centering}|}
\hline
\multirow{2}*{\textbf{Action}} & \multirow{2}{=}{\textbf{Ground Truth}} & \multirow{2}*{$\bar{E}_{M}$} & \multicolumn{4}{c|}{$L_{i}(\%)$} \\
        \cline{4-7}
        ~ & ~ & ~ & \textbf{Formal} & \textbf{Relaxed}& \textbf{Sad} & \textbf{Negligent} \\
\hline
\textbf{A01:} right arm swipe to the left
&$-$&$-$0.2354&87.50{\%}&12.50{\%}&0{\%}&0{\%}\\
\hline
\textbf{A02:} right arm swipe to the right
&$+$&0.2238&90.63{\%}&9.37{\%}&0{\%}&0{\%}\\
\hline
\textbf{A03:} right hand wave
&$+$&0.1934&75.00{\%}&25.00{\%}&0{\%}&0{\%}\\
\hline
\end{tabular}
\end{table*}

compile like this:

I only know how to centering the whole column..so anyone can help?

A:

Is this what you want? I don't the % in all cells are necessary, since it is specified in the \multicolumn{4}{c}{...}.
\documentclass[twocolumn]{article}
\usepackage[showframe]{geometry}
\usepackage{tabularx, caption, multirow, makecell, siunitx}
\renewcommand{\theadfont}{\normalsize\bfseries}

\begin{document}

\begin{table*}[!htbp]
  \setlength{\extrarowheight}{2pt}
\caption{Speech content guidance for SoHRI.}\label{tab:6}
\centering
\setlength{\tabcolsep}{4pt}
\begin{tabularx}{\linewidth}{|>{\hsize=1.6\hsize}X | >{\hsize =0.4\hsize\centering}X |S[table-format=-1.4]|*{4}{r|}}
\hline
\multirow{2}{=}[-1ex]
{\textbf{Action}} & & {\multirow{2}{*}[-1ex]{$ \bar{E}_{M}$}} & \multicolumn{4}{c|}{$L_{i}\,(\%)$}% $}}
 \\
        \cline{4-7}
        ~ & \multirow{-2}{*}[-0.2ex]{\thead{Ground\\ Truth}} & ~ & \thead{Formal} & \thead{Relaxed}& \thead{Sad} & \thead{Negligent} \\
\hline
\textbf{A01:} right arm swipe to the left
&$-$& -0.2354 & 87.50 & 12.50 & 0 & 0 \\
\hline
\textbf{A02:} right arm swipe to the right
&$+$& 0.2238& 90.63 & 9.37 & 0 & 0 \\
\hline
\textbf{A03:} right hand wave
&$+$& 0.1934 & 75.00 & 25.00 & 0 & 0 \\
\hline
\end{tabularx}
\end{table*}

\end{document} 

A:

I took a few liberties with the formatting.  Note that "Negligent" is wider than 1.2cm.
\documentclass[twocolumn]{article}
\usepackage{array,multirow}
\usepackage{showframe}
\begin{document}

\begin{table*}[tp]% table* does not support [hb], and [!] is pointless
\caption{Speech content guidance for SoHRI.}\label{tab:6}
\centering
\begin{tabular}
{| m{5cm} |c|r|r|r|r|r|r|}
\hline
\multirow{2}*{\textbf{Action}} & \multirow{2}*{\parbox[c]{1.4cm}{\centering\textbf{Ground Truth}}} &
  \multicolumn{1}{c|}{\multirow{2}*{$\bar{E}_{M}$}} & \multicolumn{4}{c|}{$L_{i}(\%)$} \\
\cline{4-7}
~ & ~ & ~ & \makebox[1.2cm]{\textbf{Formal}} & \makebox[1.2cm]{\textbf{Relaxed}}
  & \makebox[1.2cm]{\textbf{Sad}} & \makebox[1.2cm]{\textbf{Negligent}} \\
\hline
\textbf{A01:} right arm swipe to the left
&$-$&$-$0.2354&87.50{\%}&12.50{\%}&0{\%}&0{\%}\\
\hline
\textbf{A02:} right arm swipe to the right
&$+$&0.2238&90.63{\%}&9.37{\%}&0{\%}&0{\%}\\
\hline
\textbf{A03:} right hand wave
&$+$&0.1934&75.00{\%}&25.00{\%}&0{\%}&0{\%}\\
\hline
\end{tabular}
\end{table*}

\end{document}

In this solution I measure the width of "Negligent" and split up "Ground Truth".
\documentclass[twocolumn]{article}
\usepackage{array,multirow}
\usepackage{showframe}
\begin{document}

\begin{table*}[tp]% table* does not support [hb], and [!] is pointless
\caption{Speech content guidance for SoHRI.}\label{tab:6}
\centering
\sbox0{\textbf{Negligent}}% measuare width
\begin{tabular}
{| m{4cm} |c|r|r|r|r|r|r|}
\hline
\multirow{2}*{\textbf{Action}} & \textbf{Ground} &
  \multicolumn{1}{c|}{\multirow{2}*{$\bar{E}_{M}$}} & \multicolumn{4}{c|}{$L_{i}(\%)$} \\
\cline{4-7}
~ & \textbf{Truth} & ~ & \makebox[\wd0]{\textbf{Formal}} & \makebox[\wd0]{\textbf{Relaxed}}
  & \makebox[\wd0]{\textbf{Sad}} & \makebox[\wd0]{\textbf{Negligent}} \\
\hline
\textbf{A01:} right arm swipe to the left
&$-$&$-$0.2354&87.50{\%}&12.50{\%}&0{\%}&0{\%}\\
\hline
\textbf{A02:} right arm swipe to the right
&$+$&0.2238&90.63{\%}&9.37{\%}&0{\%}&0{\%}\\
\hline
\textbf{A03:} right hand wave
&$+$&0.1934&75.00{\%}&25.00{\%}&0{\%}&0{\%}\\
\hline
\end{tabular}
\end{table*}

\end{document}

