The demand for dense wavelength division complex narrowband filter film for 5G optical communication is increasing, and the research on DWDM filter film is more mature at home and abroad, but most of them adopt sputtering to form film, and there are fewer reports on the preparation of DWDM filter film by thermal evaporation, which has research significance because the deposition rate of film material is large and the time used is relatively small. However, due to the large deposition rate, the accuracy and sensitivity of the film thickness monitoring system is more demanding. In addition, the DWDM film system is highly sensitive, and if the traditional method of adjusting film thickness uniformity by means of correction plates is used, the correction plates can be deformed due to the long coating time and high temperature in the vacuum chamber, which makes it difficult to meet the control accuracy. Therefore, in this paper, when preparing DWDM filter film by thermal evaporation, the ion beam etching principle is used to correct the film uniformity in order to improve the effective coating area. The feasibility of using ion source etching to adjust the film uniformity is firstly verified by comparing the film thickness uniformity with and without ion source-assisted deposition. Based on this, the effects of ion source acceleration voltage, ion source voltage and ion source current on the film uniformity of Ta₂O₅ and SiO₂ materials are further investigated by using the control variable method. Among them, the film layer uniformity gradually becomes better as the ion source acceleration voltage and ion source voltage increase, and the film layer uniformity first becomes better and then worse as the ion source current increases. Compared with the change of ion source current value, the change of ion source acceleration voltage has more influence on the membrane uniformity. By analyzing the experimental data of the monolayer film, it is determined that the monolayer film uniformity is better when the ion source acceleration voltage is 750 V, the ion source voltage is 900 V, and the ion source current values are 165 mA and 200 mA for SiO₂ and Ta₂O₅, respectively. The optical direct monitoring method is used to monitor the film thickness during the plating of the narrow-band filter film. The signal of the real-time brightness value of the substrate is collected, and the collected brightness data is fitted using sin curves, and the evaporation is ended according to the fitting results. This method can accurately monitor the film thickness with relatively small requirements for environmental noise, and the crystal-controlled average thickness method is used to monitor the coupling layer. The inverse analysis of the experimental results is performed using Macleod film system design software, and the filter film passband ripple is improved by adjusting the ion source parameters for multiple experiments to adjust the thickness distribution of the two materials and modifying the thickness of the coupling layer. The difference in the center wavelengths of the spectral curves of the points on the final substrate that meet the technical requirements is 0.15 nm, and the center wavelengths of the spectral curves are between 1 557.26 nm and 1 557.41 nm, all in the same channel. The maximum insertion loss in the passband of each curve is in the range of 0.13 dB to 0.19 dB, which is less than 0.2 dB. The passband width at -0.2 dB is about 0.48 nm, and the passband ripple is between 0.05 dB and 0.09 dB. The bandwidth at -13.5 dB is about 0.81 nm, and the insertion loss of the reflection band of each curve is greater than 20 dB. The effective coating area is 2 123 mm² by calculating the area of the circle where the points of the spectrum meet the technical requirements.