For the needs of maritime surveillance, an optical system of refrigeration infrared thermal imager is designed, for which the detectability is not less than 23 km for an over 500t level sea surface ship (greater than 10 m×10 m). The infrared detector is determined. The parameters such as focal length, zoom ratio and aperture are determined by the distance analysis. The detailed design for the optical system is performed according to the above parameters using Zemax software optical design. The imaging quality and aberration is analyzed. The detectability is verified by field experiment, which proves the rationality of design.