It's my new PC with a new os of ubuntu.

every time when I want to install software or update apt-get, this error will appear:

First, as the most usual functions the website given:

to type these commands:

sudo rm /var/lib/apt/lists/* -vf

sudo apt-get clean

sudo apt-get update

sudo apt-get upgrade

But not worked.

Finally, I reset the system settings:

See,"download from xx."

fix it from US's  to this.

then it worked.