Mystikal -- If Music Fails ... I'M DOING PORN

Mystikal is giving music one last shot -- but tells TMZ, if he fails to make another hit on his next album ... HE'S TURNING TO PORN.

This is not a joke. Mystikal tells us, he needs to make sure the money keeps coming in ... and if there's one thing he's good at, it's having filthy sex. So porn is his plan b.

Which is either crazy ... or perfect -- because Mystikal served SIX YEARS in prison for sexual battery beginning in 2004 after forcing his hairstylist to perform sex acts on him.

But Mystikal is undeterred, telling us he already has a history in porn -- winning an AVN award in 2003 for an adult movie he made (he doesn't actually have sex in it though).

We asked Mystikal who he'd want to star in his first skin flick, and the rapper said Pinky -- adding, "She's a bad bitch. I would take her down in a second."