Using the Si substrate and the HfO₂ single layer as the examples, the power spectral density (PSD) of the optical surface was calculated and characterized. First, the method that calculates the 1D power spectral density(PSD_1D), 2D power spectral density(PSD_2D) and isotropic power spectral density (PSD_ISO) was introduced. Then, the surface morphologies of the Si substrate and the HfO₂ single layer were measured using atomic force microscopy with four scan areas, for example, 1 μm×1 μm, 5 μm×5 μm, 10 μm×10 μm, 20 μm×20 μm. Using a MATLAB program, the PSD_ISOs of the different scan area were calculated. And the PSDISO-Combined over a larger spatial frequency range was given using the geometric mean of these PSD_ISOs. The result shows that the PSD_ISO-Combined of Si substrate before and after HfO₂ coating were similar for the low spatial frequency region but quite different for the middle and high frequency region. The poly-crystallized microstructure of the HfO₂ coating is the main reason for the observed PSD difference between the Si substrate and the HfO₂ coating. The σ_ISO and σ_STD were calculated and compared. The results are quite similar, which proves the correctness of the proposed method for the PSD calculation.