At present, most of the strapdown seekers use the combination of infrared single band and single field of view to match and intercept the target, but this method is susceptible to factors such as target/background characteristic, detector performance, field of view in increasingly complex battlefield environment. On the basis of retaining the original single medium wave infrared large field of view of a strapdown seeker, a new type of IR dual band/dual field of view seeker was proposed by adding two long wave large and small field of view. By using the image fusion of medium and long wave dual band and the switching of dual field of view under long wave infrared, the difference between target and background feature information was highlighted, which can effectively improve the interception probability of missile to target. The optical system was designed with CODE V software. The whole system adopted the common aperture optical design, which had good imaging quality and meets the system requirements.