Last year,the officer came to my home for site visit for temp stay of extension (o non immd visa).They asked my way of earning living and we told them we got the interest derived from stock. He said we must provide them other way such as managing small business and stock investment should not be the only channel of earning living. I wonder if i say i have some business overseas and will they request us to show some proof?(Actually, I have 400k deposit as required in the bank and never below this amt)

There is no rule that states what your income needs to be from.

If you are using the 400k baht in the bank option there should be no need to show proof of income.

I don't think there will any questions about it when you apply for your next extension.