For the requirement of extensive temperature adaptability of infrared optical systems, a refractive-diffractive hybrid approach is applied to an optical system with 114.7 mm focal length, F/1.5 F-number, ±2.5° field-of-view in the 3.2～4.2 μm waveband. The new designed hybrid system is passively athermalized over the temperature range from -60 ℃ to 160 ℃ with very high and stable imaging quality. The theoretical polychromatic modulation transfer function (MTF) is above 0.78 at space frequency of 20 lp/mm and is near to the diffraction-limitation. The lens is compatible with uncooled staring array imager with the picture element size over 25 μm.