The Shanghai Jiao Tong University Spectroscopic Telescope (JUST) holds significant importance for astronomy in China. It will assist scientists in achieving a series of influential research results in fields such as supernova explosions, identification of gravitational wave sources, and exoplanet detection with its outstanding sky survey capability and high-precision observation technology. The 4.4 m primary mirror of JUST will adopt segmented mirror active optics technology, which could be divided into two cases: the segmented primary mirror will be in co-focus under the limit of seeing; it will be equipped with adaptive optics, and the segmented primary mirror will achieve co-phasing in the near-infrared waveband (such as 1.8 μm). This paper will focus on the research of errors in the hexagonal segments of the primary mirror under the two conditions. Additionally, separate and comprehensive error simulations were conducted, obtaining a large number of successful simulation results.