Based on requirements of infrared target detection and laser ranging, laser/ infrared optical system with common optical path and athermalization is designed. According to user requirements, the parameters of the optical system are defined as: the focal length is 150 mm, the F number is 2, the operating waveband covers 1.57 μm and 3~5 μm . A RS-RTM-31243-001-C1 InGaAs photoelectric detector is selected for 1.57 μm laser wavelength; and a 2/3 inch (1 inch=2.54 cm) detector of ULIS company is selected for 3~5 μm waveband (the full field of view is 4.2°). The evaluation results of image quality show that, in temperature range of -50 ℃~70 ℃, maximum value of root mean square (RMS) radius of the spot diagram at each field of view is 8.166 μm , which is far less than the effective receive radius of detector (75 μm ) and the energy concentration is very high at 1.57 μm laser wavelength; at cut- off frequency of 15 lp/mm, modulation transfer function (MTF) of the 3~5 μm infrared waveband are all above 0.78 for each field of view, which is closed to the diffraction-limited curve. So the system has good image quality, simple structure, and meets the user requirements of the optical overall design.