is there an option for output path of build files?
just like this way.

I want to use the plugin's option to overwrite the webpack output option.
Output them where? Seems more like it would be a Webpack question than a Babel one, but I can try to help if it's clearer what you mean.
I just want to separate my js files & css files in different folder.
for example:
"src/js " => "build/js/"
"src/css " => "build/css/"
but, now it's all in one folder, how can I do that?

I'm looking forward to your early reply.
Sorry, I don't know the right approach for that. I'd recommend using Webpack's recommended support channels from https://github.com/webpack/webpack#support
https://gitter.im/webpack/webpack is probably the best place to start.
https://github.com/webpack-contrib/extract-text-webpack-plugin/issues/694#issuecomment-362549892
Most helpful comment
my solution
https://github.com/webpack-contrib/extract-text-webpack-plugin/issues/694#issuecomment-362549892