edit file : /etc/apt/sources.list
add:
deb http://mirrors.163.com/ubuntu/ intrepid main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ intrepid-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ intrepid-backports main restricted universe multiverse
then:
# apt-get update