As the primary mirror seeing of a ground based large aperture telescope is directly related to its image quality,this paper researches the effect of environments on the primary mirror seeing.Through theoretical analysis,it points out that the size of the primary mirror seeing will increase with the temperature change between the primary mirror surface and the environments.Then using thermal analysis software Radtherm,the temperature changes and temperature distributions of a 2 m SiC lightweight primary mirror were analyzed under two different conditions of natural convection and natural flowing.Finally,the some temperature experiments on the 2 m SiC lightweight primary mirror under the working conditions mentioned above were performed to verify simulation results.The experiment results indicate that it takes about 4 hours to reach temperature equilibrium with the environment for the primary mirror in case of natural convection with initial 6 ℃ temperature difference;however,it takes only about 1.5 hours in case of natural flowing with initial 8 ℃ temperature difference.It shows the greater temperature difference but much less consumed time.These means that when the thermal control system and control method are forced to the large-aperture telescope system to perform the convective heat control,the mirror seeing will be controlled in a reasonable range more quickly and more effectively,and more telescope observation time can be obtained meanwhile maintaining much better image quality.