The effect of spectral width of thickness-monitoring system on performance of wavelength division multiplexing (WDM) filters is discussed. Unusual thickness-monitoring signal during deposition is analyzed, which is because of wide spectral width and deviation between monitoring wavelength and central wavelength of the filter. Therefore the spectral width of thickness-monitoring system must be less than half of the turning wavelength width for last two layers of single Fabry-Perot filter. This means that spectral width should be at least less than 0.2 nm for a 100 GHz filter. When the central wavelength is deviated, the thickness error occurs. The signal change regularity for high index and low index materials is shown. When central wavelength is longer than monitoring wavelength, the thickness becomes thinner; and when central wavelength is shorter than monitoring wavelength, the monitoring signal will change in unusual way. These induce a large thickness error, and make maximum transmittance drop and full width at half maximum (FWHM) increase.