In order to study the effective working distance for shipboard infrared sensors, according to the definition and features of Lambert radiation source, based on the scientific analysis for surface battleship′s infrared radiation, a calculation model was put forward for ship′s infrared radiation power on specific band range and on specific direction. The model fully considered the double property, namely being as a point source and simultaneously as a surface source, the power calculation formula was put out under different directions with the help of relative radiation table. In order to validate the model′s effectiveness, the "FLIR" infrared imaging equipment was used to carry out the relevant experiment. The results show that the calculated working distance from the model witnesses a good agreement with the experimental results. The model can serve as a basic gauge for studing the working distance of shipboard infrared sensors under bad-weather conditions.