Performance of focal-plane imaging and off-axis digital holography is studied by the way of the resolution measurement and imaging of watermark and words in pencil with the built 2.52 THz focal-plane imaging and off-axis digital holographic devices. The results validate that the common focal-plane imaging can not achieve relatively high resolution because of the long wavelength of terahertz. The imaging quality is determined by the placed position. Terahertz off-axis digital hologram can improve the resolution of the imaging system; however, the imaging object size is limited by the system resolution. It can be inferred that the focal-plane imaging is more suitable for large object and low resolution imaging.