The primary mirror of large aperture solar telescope is directly exposed to the air, the temperature difference between the primary mirror and the environment causes the mirror seeing. In order to decrease the mirror seeing, we study the influences of the temperature and the velocity of the cooling air as well as the velocity of the air knife on the mirror seeing. Combined with the primary mirror of 2.5 m called Imaging and Spectroscopic Multi-Application Telescope (ISMAT) of Nanjing University, thermal control technology is studied, and the optimization goal is mirror seeing of 0.02 arcsec. The results of numerical solution show that: the cooling air temperature and the inlet velocity satisfied the optimization goal are 283.15 K and 3.5 m/s without the air knife; the cooling air temperature, the inlet velocity, and velocity of air knife satisfied the optimization goal are 283.15 K, 3 m/s and greater than or equal to 3 m/s.