Floyd Mayweather's Dad -- FALLS HARD ... At Pre-Fight Dance Party (Video)

At least ONE Mayweather went down this weekend ... Floyd's father ... who ate it HARD during a pre-fight concert ... and TMZ Sports has the video.

Floyd Mayweather Sr. got up onstage and did his best James Brown impression to the tune of "Sex Machine" at the Fiesta Rancho Casino in Las Vegas on Thursday.

But while he was lettin' loose -- he lost his footing ... and went crashing down to the dance floor.

The good news -- like a true pro, the 62-year-old popped RIGHT BACK UP and continued to dance ... even busting out the signature James Brown "Camel Walk" dance move.