A long-wave infrared (LWIR) dual-field scanning optical system is introduced.The design model of dual-field scanning optical system is depicted.Through the oscillating mirror scanning the field of view (FOV) is enlarged.By inserting vertically a set of lens in the system the dual-FOV design for 2.5 zoom ratio is realized.The F number is 1.67 and the scanning angle of the oscillating mirror is ±6.5°.The optical system is designed by using two common optical materials Ge and ZnS.The aberration can be well corrected by use of non-spherical technology.The MTF approaches the diffractive limit and the system has fine image quality in the temperature range from -40 ℃ to 60 ℃.It can be used in the electro-optical system onboard the aircrafts and ground vehicles.