To further improve the reconstruction speed under the premise of high-resolution millimeter-wave images, a fast backpropagation algorithm based on the dimensionality reduction strategy (DR-BP) is proposed. The proposed algorithm was experimentally verified via submillimeter-wave single-input and single-output (SISO) imaging (280-320 GHz) and orthogonal array multiple-input and multiple-output (MIMO) FEKO simulation imaging (70-80 GHz). The experimental results indicate that the DR-BP algorithm is able to obtain images with less margin interference compared with the fast Fourier transform-based algorithms for SISO imaging. Moreover, the DR-BP algorithm significantly improves the reconstruction speed—60 times that of the traditional BP algorithm, as shown by the results of the experiments.