The invention discloses a mold cavity vacuum pumping mechanism of a BMC injection molding product. The mechanism comprises a flat upper mold core, a flat lower mold core matched with the upper mold core, a mold cavity formed by closing of the upper mold core and the lower mold core, a flat pad plate used for fixing the lower mold core, a flat fixing plate used for fixing the pad plate, a vertical insert embedded into the lower mold core and a vertical ejecting rod ejecting the product upwards during mold opening. The mold cavity vacuum pumping mechanism of the BMC injection molding product is reasonable in structure, and during injection molding, gas in the mold cavity can be exhausted out.