The power spectral density was investigated to analyze the seeing-limited performance of large telescope. The Karman spectral with different frequency knees and the Power Spectral Density(PSD) slope were used to simulate the atmosphere turbulence. And the Zernike polynomials was used to analyze the low order deviations. Combining the two kinds of deviation, the numeral modal was built to simulate the system. Lastly, by analyzing the PSD of actual sample and the sample combined with the numeral atmosphere turbulence, the separation of the abberations with different sourse was obtained. The results show that the proposed method can specify the performance of the large telescope under limited seeing.