Shame! Beyonce's dad is poor, broke!

Mathew Knowles -- who once made millions of dollars a year as Beyonce's manager -- just got his child support slashed because his income has plummeted in radical fashion.

Knowles asked a judge to reduce the $12,000 monthly child support he pays to baby mama Alexsandra Wright, with whom he has a 3-year-old son.
  

Knowles -- who made $3 million a year before his daughter fired his ass in 2011 -- now says he pulled in a relative paltry $127K in 2013.  

And, Knowles says, even though his income has been slashed, his expenses remain sky high -- $51,000 a month.  To make up the shortfall, he's liquidating his assets.

The family court judge apparently validated his financial plight, and reduced his child support to $2,485 a month.

Not great news for Alexsandra, who's been getting $300 a month in food stamps.