When a ship sails on the sea, a clear wake with a very long lasting time will be formed behind it. The length of the wake can extend to several thousand meters. The detection of ship wakes by infrared imaging has been a military reconnaissance technology to be developed prior in each military power in the world. Firstly, in view of temperature resolution and spatial resolution, the infrared detection condition of ship wakes is derived. Secondly, on the basis of infrared temperature measurement principle, the expression of equivalent temperature difference between the ship wake and the sea surface in non temperature stratified seawater is obtained. Finally, on the basis of the threshold signal-to-noise ratio and equivalent band number, the infrared detection probability of the ship wake is derived. The research results show that the ship wake can be detected in non temperature stratified seawater only when the detection zenith angle is larger. Moreover, the detection probability will increase with the increase of detection zenith angle.