'I study Economics' or 'I'm studying Economics'?

Both are perfectly acceptable English.

It would, perhaps, be more usual to use the first along with a qualifying phrase such as "at the London School of Economics" making the whole statement "I study Economics at the London School of Economics".

I study economics is a usage more common when the context of the discussion is already about studying, and what is most relevant is the information about what you study.

I'm studying economics would be used when immediately before that statement, you first established that you are in school.

When the

"I study Economic" is a time general statement. It does not refer to the immediate moment. It can mean at any point in your life.

"I'm studying Economics" is a time specific statement. It means that right now you are studying economics. No more general than being an Economics major in a current course of college but preferably it means there is an open economics book in front of you.

I study economics is the right thing to say. I'm studying economics is wrong because you are not talking about studying at all right now, but yourself.

I'm studying Economics is he right anwers, as it is the present continuous :)

You can use either depending on the sense in which you are using them.

both are correct