在表格和插图中加入【 [h] 】即可，如下：
\begin{table}[h]
\caption{The summary of the notations frequently used in this paper}
\label{tab:notations}
\setlength{\tabcolsep}{3pt}
\begin{tabular}{|p{50pt}|p{180pt}|}
\hline
Notation&
Definition\\
\hline
注意，这里用大写H不行，要用小写h。