Aiming at the problem of small-field, central obscuration and low anergy of reflective optical system, the dual-mode beam positioner optical system of a folded/diffractive co-aperture infrared dual-band positioner was proposed. Based on achromatic theory and MRTD model, the equations of wavelength band achromaticity and action distance were deduced, respectively. With the property of a common focal length in the dual-band optical system, dual-band simultaneous detection and consistency of target information identification were achieved. The parameters of the folded/diffractive co-aperture infrared dual-band positioner optical system were as follows: wavelengths are 3.4-4.8 μm and 7.7-9.5 μm, pitch/yaw is -26°-26°. focal length is 115 mm and F number is 2. The results show that thermal difference eliminated in the temperature range of -40-60 ℃, the optical transfer function is close to the diffraction limit.