In order to improve the imaging performance of dual-band optical system, considering the characteristics of visible light and MWIR, diffractive-refractive dual-band co-path and parfocal without compensation optical path optical system was designed. The four lens achromatic and focal length compensation expression between wave band were derived. The system used four lenses and introduces binary diffraction surface by properly matching the optical system power to achieve common optical path. The focal lengths were consistent between the visible and infrared, which improves consistency of dual-band target information. The optical system operating wavelength is 0.38-0.76 ?滋m and 3-5 ?滋m, the focal length of the whole optical system is 90 mm, the field of view is 0.5° and the F number is 3, respectively. The dual-band athermal optical system with the temperature of -40-+60 ℃ was designed with the use of optical passive. The results show that the designed system structure is simple and the size is small, has an excellent in overall image quality.