Atmospheric infrared radiation transmittance is the basis for infrared radiation characteristic measurement and infrared radiation inversion calculation. The atmospheric horizontal transmittance at long distance near the sea surface is greatly affected by factors such as environmental meteorological conditions and horizontal transmission distance. Based on the independently developed offshore target infrared radiation characteristic measurement system and atmospheric radiation transmission measurement system, this paper uses the sea surface sawing method to conduct long-distance medium-wave infrared measurement experiments, and uses the radiation transmission software CART to calculate the atmospheric transmittance to realize the correction of the atmospheric transmittance of long-distance medium-wave radiation transmission. This study has certain reference significance for the measurement of offshore long-distance target characteristics.