Short-medium wave infrared detectors can work both in the low light night vision band and medium band at the same time, which can meet the requirement of complex detection environment. Infrared detector are widely used in military and civil fields. In order to improve the precision of infrared detector and shorten the response time, it is necessary to develop a wide band film with high transmittance that meets the system requirements in the short-medium band. The evaluation function of the variable scale algorithm was optimized based on the Baumesiter anti-reflection membrane design theory. A new weighted evaluation function model was established to design the membrane system structure with low sensitivity and high tolerance in the water absorption band of 2.6-3.3 μm. The influence of different ion source assisted deposition parameters on the spectral characteristics of MgF2 was studied by optimizing the preparation technology for the water absorption band. Meanwhile,a step annealing process was used to obtain an effective method to reduce the water absorption. Finally,the spectral transmittance of the prepared film was greater than 96.5%.