To ensure the quality and efficiency of manufacturing and detecting large aperture optical systems, a method to improve the seeing metrological accuracy was proposed. By determining the wavefront slope with electronic autocollimators and a flat mirror, the relationship between slope information and mirror seeing was deduced. With the purpose to enhance the accuracy and to exceed the limit of only one autocollimator, three autocollimators were used to calibrate each other. To decrease errors, a hexapod platform was adopted to support the cube mirror to improve the accuracy to 0.01″. Moreover, hexapod platform errors induced from the test environment was estimated by the related principle of frequency response function. Finally, the normalized Point Source Sensitivity (PSSn) was deduced to evaluate the testing results. The simulation and experiment were performed. For mirror seeing, the slope power spectra and original power spectra in two different directions were calculated, and both PSSn in two directions are 0.999 based on the relation between power spectra and PSSn. This method can be used to quantitatively predict the mirror seeing of primary mirrors under actual working conditions of the telescopes.