In view of the scarcity of terahertz astronomical point source targets, this paper studies the method of using a small optical telescope coaxial with terahertz antenna to assist the pointing measurement of terahertz telescope and establish the pointing error correction model. Based on the 1.2m oblique axis terahertz antenna of Purple Mountain Observatory, the experiment of optical aided pointing measurement is carried out. A 100mm aperture refractive optical telescope mounted on the antenna back frame is used to obtain the pointing measurement accuracy better than 2$''$. In addition, by analyzing the structure of the slant axis antenna and the error sources of atmospheric refraction and local star time deviation, a slant axis optical pointing correction model with 23 error terms is established, and the fitting accuracy of about 3$''$ is achieved. Finally, with the help of high-precision digital photogrammetry, the photoelectric axis consistency is calibrated, and its influence on the pointing model accuracy is discussed. The research results of this paper will provide technical reference for the 5 m Dome A Terahertz Explorer (DATE5) and other terahertz telescopes in pointing measurement and correction.