Base on the convolution model of material removal and power spectral density function in optical manufacturing, a mathematical model named smoothing spectral function was established. A Zerodur plate mirror with diameter of 620 mm was polished using Rigid Conformal tool. By calculating the smoothing spectral curve, the error restraint ability of polishing process can be expressed as a normalized and nondimensional spectral curve. The calculated result indicates that the error restraint ability of polishing process can be evaluated by the value of smoothing spectral function curve.