I am doing my bsc... I want to step in safety field... Can i do nebosh with my graduation or should i wait and do it after my graduation?

It is not mandatory to have a Bachelor Degree to attempt NEBOSH IGC.

But. Even if you have NEBOSH IGC, your academic qualification is always important for your career. I suggest you to concentrate on your BSc for now. Since NEBOSH IGC is 15 days class session, you can go for it later.