Mixed matrix membranes (MMMs) provide an efficient implementation that overcomes the bottleneck of individual inorganic or polymeric membranes for gas separation, but still face challenges. Here, we put forward a strategy of using in situ synthesized polymer grafted metal organic frameworks (MOFs) to eliminate both interfacial and carrier problems existing in the MMMs. Specifically, polyethyleneimine (PEI) grafted ZIF-8 (PEI-g-ZIF-8) was synthesized in situ by using Zn²⁺, 2-methylimidazole, and hyperbranched PEI through a rapid method at room-temperature. The chemical composition and porous structure of PEI-g-ZIF-8 were investigated by Fourier transform infrared spectroscopy, powder X-ray diffraction, N₂ adsorption–desorption measurement, etc. The results indicated that the synthesized PEI-g-ZIF-8 achieves an enlarged pore volume and aminated surface. The improved interfacial compatibility between PEI-g-ZIF-8 nanoparticles and poly(vinylamine) (PVAm) was confirmed through high-sensitivity differential scanning calorimetry and zeta potential measurements. According to the CO₂/gas separation tests, the MMMs incorporating with PEI-g-ZIF-8 achieved 1990 GPU of CO₂ permeance and 79.9 of selectivity for CO₂/N₂ (15/85 vol%) separation, and 1790 GPU of CO₂ permeance and 40.7 of selectivity for CO₂/CH₄ (10/90 vol%) separation, under 0.30 MPa gas pressure.