The principles and calculation methods of the process capability index ( C_pk) were introduced. Based on statistical analysis on the cutoff wavelength of a certain bandpass filter, the Cpk of the thin film filter was calculated to be 1.0～1.33 from the distribution of the points of 5% transmittance. The methods for increasing the Cpk of the thin film filter deposition are put forward, such as reducing the relative standard deviationsδ, optimizing the technological conditions, reducing the deviation of the technological parameters between the distribution center central μand the standard central value T0.