In order to achieve the quantitative analysis of infrared filters’ radiation characteristics, we propose an equivalent emissivity inversion method for infrared filters in the cutoff band and establish the basic mathematical model of the inversion method. Firstly, in two cases, whether placing the infrared filter in the light path or not, we conduct the radiometric calibration for infrared imaging system and then calculate the self-radiation which is caused by infrared filter in the cutoff band based on the calibration results. Secondly, based on the mathematical model of radiation from the limited surface source to the micro surface source in polar coordinates, we inverse the equivalent emissivity of the infrared filter in the cutoff band. According to the basic mathematical model, we carry out an inversion experiment of a mid-wave infrared (MWIR) filter, in two cases that the integration times are set to 0.8 ms and 4.0 ms. The result of the equivalent emissivity, arithmetic average of inversion values obtained in the two cases, is approximately equal to 0.420, so that the equivalent reflectance is approximately equal to 0.580. The relative difference value of the two experiment cases is small, approximately equal to 1.9%. We conduct the reflection imaging experiment of the MWIR filter. It is proved that the MWIR filter has strong reflective ability of radiation in the cutoff band. What’s more, the conclusion of the inversion experiment is verified qualitatively by the phenomena of the reflection imaging experiment.