With the increase of the aperture of the telescope, the inertia of the primary mirror increases sharply. Both of the temperature difference with optical surface and ambient air and turbulence fluctuation at thermal boundary layer result in heavy mirror seeing, directly influence the optical quality. A method based on computational fluid dynamics and optical path difference integrated calculation was proposed to evaluate the optical quality due to turbulence fluctuation at thermal boundary layer. To verify this method, the thermal boundary temperature distribution of a 3.0 m aperture primary mirror was simulated and calculated in different temperature warmer optical surface in natural and forced convection, respectively. Then, the temperature field was transformed to be refractive index field by corresponding equations. The optical performance of thermal boundary layer was calculated by optical path difference integration at the refractive index field. The results can quantitatively describe the effect on optical quality from the thermal boundary layer, which has made up for the deficiency of the existing seeing test. Meanwhile, it is verified that the existing astronomical observation requires the temperature difference between the primary mirror and the ambient air less than 2 K. The primary mirror thermal control system makes the optical path difference in forced convection decrease by an order of magnitude than that in the natural convection, which significantly improves the primary mirror seeing. Furthermore, it is indicated that the thermal control at optical surface is of great significant to improve the image quality.