In terahertz off-axis digital holography, the zero-order diffraction will still cause bad effects on the reconstructed image, as a result of short recording and reconstruction distance. Using three image quality objective evaluation indexes, the reconstructed images are compared and analyzed. The images are obtained by using the complex-wave retrieval method and the Laplacian operator method to eliminate the zero-order diffraction in 2.52 THz off-axis digital holography. Besides, the simulation target is designed with resolution chart in the actual imaging experiment. The hologram is simulated according to the intensity distribution of illumination light and reference light got from the real experiment, then the angular spectrum method is used to reconstruct the hologram. Finally, the corresponding processing is also performed to the hologram in the real terahertz digital holography experiment, and the results fit well with the simulation conclusions. Both methods have good suppression of zero-order diffraction. Compared with the Laplacian operator method, the complex-wave retrieval method has a higher brightness attenuation rate of zero-order diffraction, and the reconstructed image of it has higher peak signal to noise ratio overall. What′s more, it has higher signal to noise ratio in 0.2 mm bright vertical stripe area with high resolution, and the image quality is better.