modify the software source:
The software source is a place where several free application for linux is stored, you don't compile applications by yourself when using application directly. The configuration file is "sources.list" ,which is placed in "/etc/apt/sources.list".Raspbian has a default software source in the "sources.list".Because applications is in the foreign server,the download speed is so slow.But some institutions offer their download link,which download application from foreign server to their servers. The domestic links are as follows:
University of Science and Technology of China : http://mirrors.ustc.edu.cn/raspbian/raspbian/
Tsinghua University: http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/
Zhejiang University: http://mirrors.zju.edu.cn/raspbian/raspbian/
List of offical certified Raspbian software source
For example,you can note the original link and add the following link:
deb http://mirror.sysu.edu.cn/raspbian/raspbian/ jessie main contrib non-free
deb-src http://mirror.sysu.edu.cn/raspbian/raspbian/ jessie main contrib non-free
Finally,excute the following command:
$ sudo apt-get update