Dewar window is one of the core components of infrared detectors. Its welding quality has a decisive influence on the vacuum maintenance time and long-term operation stability of the detector. In this experiment, two preformed solder rings are introduced for vacuum brazing to replace the traditional manual SAC305 solder paste soft soldering process, and the appearance quality, airtightness, void rate and welding strength of the dewar window are studied. The experimental results show that when the SnAgCu composite solder ring with an intermediate layer of Ni skeleton structure is used for vacuum brazing, the leakage rate of the dewar window can reach a high airtightness requirement of less than 1×10^-12 Pa·m³/s, and the void rate can be reduced to 1%. The tensile strength of the dewar window is tested. The results show that when the process parameters are 250 ℃, 5 min, and 0.32 MPa, the tensile strength reaches a maximum value of 66.73 MPa; compared with the traditional manual SAC305 solder paste soft soldering joint, it increases by 10.19%. At the same time, this method can improve the consistency of the process and greatly improve the production efficiency, which has a certain guiding significance for the mass production process of the dewar windows.