目的：希望\cite后直接自动显示li et al. [1]，而不是每个引用都手动拼写作者名字。

引用包及语句：

...
\usepackage[numbers]{natbib}
...
\begin{document}
...
The grayscale image can be colorized by propagating the scribble from users. \citet{Levin2004Colorization} proposed a colorization method, which is based on the user-specific scribbles. The main idea is to solve the cost function of the luminance difference between a special pixel and its contiguous pixels using least-square optimization. While this method only considers the luminance between pixels, the color bleeding may appear at the boundaries in original images.
...
\bibliographystyle{plainnat}
\bibliography{IEEE_Trans}
...
\end{document}

效果：

需要注意的点：

1. 导入包\usepackage[numbers]{natbib}，必须有[numbers]不然在文中显示的效果就会是作者加年份。因为natbib包默认是author-year的风格。
   
2. 使用\citet{}进行引用，如果使用\cite{}最后只会出现序号。
3. 在导入参考文献bib文件的前面添加参考格式\bibliographystyle{plainnat}。

参看资料：
Why do I get (author?) when I use \citet with natbib?

怕内地访问不了，截取了有用信息。