Yes, it's absolutely fine to say 'I'm majoring in xxx'. We are using 'major' here as a transitive verb in the present continuous.

I think you can't say I am majoring in subject but I am majored in subject. because when major + subject, it should be used in passive form :)

DON'T use the passive form. You just need to say' I major in International Trade'. 'Major in' is a phrase. You can refer to Oxford dictionary. And I say ' major in ‘ to my foreign teachers all the time, it's correct. But ' I am majoring in XXX' is also right. It depends on different occasions. But i never say that.