Assume that columns B, C, and D are of the p type with usable widths given by the length parameters \colB, \colC, and \colD. In your example code, these lengths are equal to 2cm each. The total widths of columns B, C, and D are, however, not equal to 2cm. Instead, each column's total width equals 2cm+2\tabcolsep, where \tabcolsep is the parameter that governs the amount of whitespace inserted to the left and right of a column. (In the standard LaTeX document classes, the default value of this parameter is 6pt.)

The combined width of columns B, C, and D -- not counting the widths of the vertical bars to the left of B and to the right of D -- is 

\colB + \colC + \colD + 6\tabcolsep + 2\arrayrulewidth 


(I'm assuming, implicitly, that the array package is being loaded. If that's not the case, then one should omit the 2\arrayrulewidth term.) You can probably guess that 2\arrayrulewidth represents the combined widths of the two interior vertical bars -- those separating B from C and C from D. The default value of this parameter is 0.4pt in the standard document classes.

The usable width of the three columns is a bit smaller, since we mustn't impose on \tabcolsep at the left-hand edge of B and at the right-hand edge of D. It is thus given by 

\colB + \colC + \colD + 4\tabcolsep + 2\arrayrulewidth


One can create a length variable called, say, \combinedlength and set its value to the expression above. The contents of the external file table.tex would thus be given by

\hline
A &amp; B  &amp; C &amp; D  \\   \hline
E &amp; \multicolumn{3}{p{\combinedlength}|}{This is generated by excel2latex macro. I want to let it auto wrap according to the width of (B+C+D) }   \\      
\hline


The full MWE:



\documentclass{article}
\usepackage{array,filecontents,catchfile}
\begin{filecontents*}{table.tex}
\hline
A &amp; B  &amp; C &amp; D  \\   \hline
E &amp; \multicolumn{3}{p{\combinedlength}|}{This is generated by excel2latex macro. I 
want to let it auto wrap according to the width of (B+C+D) }   \\      
\hline
\end{filecontents*}

%% Define some length parameters and set their values
\newlength\colB \setlength\colB{2cm}
\newlength\colC \setlength\colC{2cm}
\newlength\colD \setlength\colD{2cm}
\newlength\combinedlength 
\setlength\combinedlength{%
    \dimexpr\colB+\colC+\colD+4\tabcolsep+%
    2\arrayrulewidth\relax}

\begin{document}
\section{ Input the body of table from a separate file}

\CatchFileDef{\mytable}{table.tex}{}% table.tex &gt; \mytable
    \begin{tabular}{|p{2cm}|p{\colB}|p{\colC}|p{\colD}|}
    \mytable
    \end{tabular}%
\end{document}


How to specify the multicolumn width in table header?
Focus and blur have a reason in a picture when they come together. Blur alone is just a loss of an important element in the photography art, unless it has a reason, like obscuring nudity or creating an additional value... but what is the value of an entirely blurred picture?
