A filling roll bending process of a 2A12 aluminum alloy integral panel with polypropylene as filler was studied. The operating effect of filler was revealed, based on analytical, numerical, and experimental methods. The results show that the position difference of the neutral layer between the ribs and the web of the integral panel leads to the error of generatrix straightness, and the filler can reduce this difference. A certain range of the width gap (1-3 mm) between filler and rib can avoid the instability of the rib and the curl of the filler. The height gap between filler and rib determines the compression degree of filler. The best pressure transmission effect can be achieved as the filler is still higher than the rib (1 mm) after compression. The elastic modulus of filler slightly affects the forming accuracy of the integral panel within a certain range (450-1300 MPa). Still, the filler transfers insufficient pressure under lower elastic modulus (50 MPa) and limits the deformation of the integral panel under higher elastic modulus (20 GPa). Experimentally measured generatrix straightness verified the operating effect of filler.