The formula of air refractive index used in this paper for optical trace computing is described in detail. Following the changing of image quality of spherical mirrors with different R is analyzed caused by air temperature layered in horizontal and vertical testing cases. The temperature gradient effects for off-axis near parabolic segment with Φ1.5 m, off-axis length 12 m are calculated and analyzed in horizontal and vertical test status. Finally, the influence of image quality by air temperature gradient distribution for large parabolic mirror with aperture Φ4 m, focal ratio f/2 with the states above mentioned is calculated and simulated. The results above all indicate that, in general, the influence level for image quality in horizontal test is much greater than vertical case, moreover, the air temperature factors should be considered in vertical optical testing in large aperture and fast focal ratio parabolic testing.