In the field of optical fabrication, the power spectral density (PSD) is used to characterize the error spectrum. However, the PSD is the statistical information of surface error, which is not as intuitive as peak-valley (PV) and root mean square (RMS). In order to analyze the relationship between PSD and process parameters, based on the definition of PSD, the influence of different parameters of random surface contour on optical PSD was analyzed, and summarized the key control points of PSD. Then, the PSD curves processed under the typical paths of numerical control polishing were analyzed on flat glass. The results show that the PSD is correlated with the amplitude and frequency distribution of random contour, and the phase has almost no influence on it. When the RMS is close, the slope of PSD linear fitting and RMS Slope decreases with the increase of the auto-correlation length of the random profile. The short-range machining path can effectively suppress the peak value of PSD curve compared with the long-range ordered path, which makes the optical element meet the requirements of spectrum suppression.