A novel strategy was designed for the preparation of a long-chain branched polypropylene (PP) with improved foamability via reactive extrusion in the presence of supercritical CO2 (scCO(2)). Benzoyl peroxide was used as a radical initiator and trimethylolpropane triacrylate (TMPTA) was applied as a polyfunctional reactive monomer during extrusion. Fourier transform infrared spectroscopy and high temperature GPC confirmed that TMPTA was grafted onto PP chains, and the presence of scCO(2) promoted the grafting and branching reactions, and hindered polymer degradation. A possible mechanism was proposed to explain the effect of scCO(2) on the branching reactions. In addition, rheological behavior of pure PP and modified PP samples was studied to investigate the effect of long chain branching of PP on the melt viscosity and strength, and foaming behavior was studied to confirm the subsequent effect on its foamability. It was found that the long chain branching increased the melt viscosity and strength of modified PP samples, which favored the foamability, and that the foaming windows were expanded in the presence of scCO(2). Thus, it provided an advanced foaming approach via preparation of long-chain branched PP through reactive extrusion with scCO(2) both working as the reactive medium and the foaming agent.