Abstract: A three-dimensional (3-D) image reconstruction algorithm based on mixed-domain is developed for terahertz holographic imager. Matched filtering in the spatial wave-number domain is employed to focus images in the vertical direction. Combined with back-projection technique used in the other two directions, the algorithm can focus images perfectly without complex 3-D Stolt interpolation computation compared with the frequency-wavenumber algorithm. Besides, motion compensations in the horizontal and range directions get easier to handle. The computational cost of the proposed algorithm is lower than the traditional 3-D back-projection algorithm owing to the FFT-based reconstruction in the vertical direction. Finally, some simulation and experimental imaging results are given in the 0.2 THz band to validate the algorithm.