In testing the 100 GHz dense wavelength division multiplexer (DWDM) angle-tuned filter with low polarization dependent loss, the insertion loss and bandwidth broadening of the transmission optical spectrum will become more and more serious as the increase of incident angle. It is not coincide with the theoretical design. According to the theory of the multiple-beam interference, the theoretical analysis model is built up and simulated. The simulation results show that the transmission facular of the thin film filter will broaden obviously. It cause a serious spectrum distortion because the collimator at the incepting port cannot collect all the output beams. According to the analysis results, one kind of facular-shaping fitting used the upright angle prisms is designed and fabricated in the paper. It can empress the elliptical facular into a nearly circular shape in one dimension. The experimental results show that it can effectively reduce the insertion loss and the phenomenon of half bandwidth broadening. Using the spot-shaping fitting, the tunable range of the filter is about 20 nm which coincides with the theoretical design.