http://blog.sina.com.cn/s/blog_5e36b0610100lwjk.html

step 1 :
\newcommand{\tabincell}[2]{\begin{tabular}{@{}#1@{}}#2\end{tabular}} 

step 2:
& \tabincell{c}{first line\\second line} &
也可以这么用:
& multirow{3}*{\tabincell{c}{first line\\second line}}