Based on the analysis of the shadow zone and the padding methods of the Radon domain generated by circular cone-beam CT scanning, this paper proposes a shadow zone padding method by distance-weighted interpolation. The method uses the known boundary data to interpolate the missed data inside the shadow zone with a distance weighting function. Simulation results show that the proposed padding method shows some advantages over the constant padding technique in terms of data-padding accuracy, artifacts reduction and image reconstruction for large cone-angle cone-beam scanning. The data padding method is particularly useful in Grangeat reconstruction algorithm. It enlarges the applicable scope of circular cone-beam tomography.