The surface accuracy of the reflector antenna directly affects the antenna efficiency, and is one of the key performance indicators of the telescope. Near-field radio holography, which features the advantages of high measurement accuracy and convenience, is one of the most commonly used surface alignment methods for millimeter and submillimeter wave radio telescopes. Near-field radio holography under Cassegrain mode can measure the overall deformation error in the optical path of the telescope. The slant-axis mount is more adaptable to the extreme site environment of terahertz telescopes, and can provide better thermal protection and control for the overall telescope. However, the unique rotation characteristics of the slant-axis antennas will introduce additional error factors during the holography measurements, and devoted study on the near-field holography of slant-axis terahertz antennas under Cassegrain mode has not been reported. In this paper, the influence of the reference receiver position and diffraction of the secondary mirror has been analyzed. The near-field holography measurements have been conducted on a 1.2 m slant-axis terahertz antenna. The repeatability of the experimental system has been evaluated to be better than 2μm RMS (Root Mean Square), and the comparison between near-field holography and photography has been performed to verify the correctness of the error analyses and corrections.