OS (and version)? Game version? Does it happen every time? Does it still happen if you use a web browser to download from mods.factorio.com? If it happens every time, both in game and when using a web browser, then my first guess is a poor internet connection to wherever mods.factorio.com is hosted...

Please help me

When I download mod from the game, the download speed is very slow. Please solve this problem.

I am in China.