In order to overcome the out-of-focus phenomenon caused by the harsh and complex aeronautical environment,the automatic focus module was introduced in the system.Mid-wave infrared image sequences in different out-of-focus were collected and analyzed on the test platform,and then the parameters of sensitivity,consistency,stability and calculating speed of some focusing evaluation functions in spatial domain were tested and validated to find the suitable function for this infrared imaging system through experiments.In order to improve the SNR of the focusing evaluation function,a method based on image preprocessing is proposed to ensure the function’s monotonic property.And then,a climbing search strategy of rapid auto-focusing combining rough focusing and fine focusing was formulated.Firstly,the function was used to roughly focus with large step to shorten the time and then it was used to finely focus in a narrow step to ensure accuracy.Based on mid-wave 640×512 focal plane array cooled detector,the Cassegrain catadioptric optical system with focal length 300 mm,F/2 was set up.The design has been realized on the auto-focusing hardware platform of the infrared imaging.By experimental verification,the auto-focusing function module can ensure real-time and precision,and meet the infrared imaging system requirement with large noise.