nick_qin the API is one (essential) part of Flarum. You can't use the API without having a working Flarum install.
What do you mean by master station ? An existing application ? Or just your localhost computer ?
If you want to integrate with an existing application, you can implement single sign on via an oauth provider, using Laravel Passport and Flarum Passport on Flarum for example, so your users can login on the forum via the existing login flow.
If you want to integrate discussions into your own pages, you can have a look at the Embed core extension, but I don't think it's been updated for beta8 yet.
For a transparent single sign on where the Flarum login links are never used, have a look at the existing Wordpress extension, it might give you a starting point.
If you want to take Flarum apart and reuse some features only inside another PHP app, feel free to try, but Flarum hasn't been made with that goal in mind. Also make sure you respect the licensing in that case.