A W-band stepped-frequency three-dimensional near-field surveillance imaging system together with an effective radar imaging algorithm is presented. W-band signal is able to penetrate common clothing barriers to form an image of a person as well as any concealed items. Performing the image reconstruction procedure in the frequency–wavenumber domain, the algorithm is able to completely compensate the wavefront curvature in the near field through interpolation process. The interpolation relationship in frequency–wavenumber domain between sampled data and desired data was demonstrated. Moreover, the difference between algorithm model and experimental data are revealed. High range resolution and lateral resolution were obtained by emitting wideband stepped frequency signal and adopting high operating frequency. The cross-range resolution of the imaging result is better than 5 mm. The scheme of imaging system is described in detail along with a set of imaging results to show its superior imaging precision and sensitivity compared with existing Ka-band equipment.