Blac Chyna is privately calling BS on Tyga for pretending to be supportive of her engagement to Rob Kardashian … we’re told her ex is really trying to destroy her life.

Sources directly connected to Chyna tell TMZ, Tyga has gone to great lengths to drive a wedge between their son, King, and her. We’re told he’s filed legal docs trying to wrestle custody from her and will not back down.

Our sources say Tyga will not even speak to Chyna … their only direct contact is email. We’re told he changed his cell and won’t give her the new number. Most of the parenting decisions are made with their nanny as the go-between.

What’s more, our Chyna sources say Tyga and his crew have been on a campaign to destroy her reputation, calling her a “drunk whore,” “deadbeat mama” and “nasty bitch.”