On the basis of the optical thin film interference theory, a novel thin film multi-frequency filtering technology applied to CWDM filter, a comb-like filter, which is not applied to select single signal channel from the multi-channel, but to separate the multi-channel into two different sets of channels, is proposed in the article. According to the central wavelength and the width of pass-band of 8-channel CWDM system that were defined by the ITU-T, two kinds of thin-film filters for 8-channel CWDM system have been designed by the technology. A specific expression for these thin film structures has been described and the parameters of these structures have been analyzed and optimized. The transmission curves of the designed structures show that the design results are agreement with that of the theoretical calculation well. We can implement the function of the 8-channel CWDM using two above-mentioned thin-film filters, which make the CWDM system low-cost, simpleness and easy collimation. These thin film structures have the advantages of good stability, arbitrary wavelength spacing between the adjacent channels, perfect rectangular shape of the pass-band, less layers, low cost and easy encapsulation, which would make it more attractive to use in CWDM system and other fields.