The simulation study of the filter's optical character affected by the In-situ optical monitoring the layer thickness and its deviation during deposition process for the optical narrow band-pass filter thin film was reported. Both of the ideal monitor trace and the simulation of optical parameters distributing curve of the 4-cavity DWDM filter were given. The simulation results are very useful in the practice of high quality optical filter thin film deposition. The filters are designed to meet specific 100GHz,200GHz DWDM and CWDM bandwidth requirements, and the three and four cavity narrow band-pass filters are produced for the industry application.