The reconstructed image noises of THz in-line digital hologram are higher, especially the background noises, which seriously affect the overall effect of the image. To realize image smoothing, this paper proposes the restoration methods on two-dimensional reconstructed images of THz Gabor in-line digital hologram by combining opening-closing-by-reconstruction in mathematical morphology with non-local means or median non-local means filter. Digital processing and comparative analysis for the real reconstructed image of the target with lower resolution are carried out. The images processing results show that the proposed composite algorithm can remove most of the noise, leading to image smoothing and signal-to-noise ratio improvement, and the effect is obviously better than seperately using any one single filter in composite algorithm.