The sea-sky atmospheric model is proposed based on the sea-sky infrared image, and the contrast transfer function of the atmospheric environment on the imaging of infrared thermal imaging system is analyzed. Taking the infrared thermal imaging system as an example, the atmospheric contrast transfer function of the slant path in the sea-sky atmospheric model is calculated. The effects of the observation distance, atmospheric density, target altitude, atmospheric path radiation, and path zenith angle on atmospheric contrast transfer function are analyzed. The experimental results show that the observation distance, atmospheric density, atmospheric path radiation, and zenith angle of the atmospheric transmission path are inversely proportional to the atmospheric contrast transfer function. The results can be used to estimate the atmospheric impact during the practical application of the infrared thermal imaging system, and provide the basis for improving the imaging performance of the infrared thermal imaging system.