用\shortstack

\begin{table}
\caption{the caption}\label{tab1}
\centering
\begin{tabular}{|l|l|}
\hline
\textbf{Method}  & \textbf{Details} \\
\hline
method1 & \shortstack{the long sentence1 \\ the long sentence2}\\ 
\end{tabular}
\end{table}