Foxx is starring in an upcoming biopic about the boxing legend -- he'll play both young Mike and older Mike. He's been working on the project for years.

But, the production seems to be picking up steam -- and during a recent episode of "Catching Up with Mark Birnbaum," Jamie revealed how he's transforming his body to look just like Tyson.

So, how's he doing it?

Foxx says he has a daily workout regimen of 60 pull ups, 60 dips, 100 push ups ... "It's changing the body. Top half."

As for lower body? There's a different plan for that.

"I ain't got no legs, I ain't got no calf muscles so we're gonna have to get some prosthetics for that."

Foxx says he'll be 216 pounds when shooting begins (he starts as young Mike) -- and then to play Old Mike, he'll "balloon to 225, 230 which will look like 250 on screen."

Jamie's been super excited about the project for years -- and even previewed the opening scene on HOT 97 back in 2015.