Rae Sremmurd's Slim Jimmy -- Slices Open Leg ... Takes Epic Bloody Pic

A member of Rae Sremmurd literally had his performance cut short when he tore open his leg on a jagged piece of glass -- the video of the accident is gnarly, but his injury is super GNARLY!

It all went down at the Governor's Ball in NYC on Friday ... the "No Flex Zone" duo was slaying when Slim Jimmy jumped from a speaker on to a spotlight, which toppled over. Slim's leg got sliced ... apparently by the shattered lens.

During the ambulance ride ....Slim took a ridiculously gruesome shot of his leg, and then decided to post it. We'll just say -- viewers beware.

Before leaving the venue, Jimmy got on the mic and told the crowd, "I got a big ass piece of glass in my leg, so if you got that video, put that sh*t on Instagram."

Well done, audience.