The invention discloses an automatic testing method and device of a minimum detectable temperature difference of an infrared imager. The method comprises a step of adjusting a gain of the infrared imager to the maximum, setting a large temperature difference such that a round hole target is clearly imaged, reducing the temperature difference such that the imaging quality of the round hole target is reduced to be M times of clear imaging, and recording a temperature difference value at that time, a step of continuing to reduce the temperature difference such that the imaging quality of the round hole target is reduced to be M times of clear imaging again, and recording a temperature difference value at that time, and a step of calculating the minimum detectable temperature difference according to the two temperature difference values. The device comprises an optical platform for bearing all equipment, a temperature control module for controlling an environment temperature, a differential blackbody source module for emitting target infrared radiation, a target wheel module for allowing a differential blackbody source to form a specified shape and frequency, a differential blackbody source temperature controller for controlling the temperature of the differential blackbody source, a parallel light tube used for controlling and converting the shape of infrared radiation, and a computer for controlling each module.