A new phononic crystal with the graded supercell configuration is proposed to broaden the Bragg scattering band gaps. The graded structural design can merge adjacent multiple band gaps into an extremely broad one. The proposed phononic crystal is made up of a periodic arrangement of supercells, and the supercells are composed of unit cells with graded structural parameters. The mechanical model of the graded phononic crystals is established based on the transfer matrix method to investigate in-plane elastic waves propagating and band structures of the periodic system. Modal analysis shows that the mechanism for the broadening of band gap is that the graded supercell configuration breaks some symmetries of the phononic crystal, resulting in the opening of the Dirac cone and creation of new band gaps. The effects of the main structural parameters related to graded supercell design on band gap broadening are studied by simulation and verified by the experiment. The present study is beneficial to the design of new functional materials with broadband vibration isolation performance.