In the new generation of infrared countermeasure technology, it is necessary to couple the infrared high peak power laser into the infrared energy transmission fiber for transmission. Aiming at the problems existing in fiber coupling of infrared wide-band and high peak power laser, this paper designs an achromatic coupling optical system in which the fiber end cap participates in focusing. It can couple 2.1-4.6 μm infrared lasers with high peak power, and the fiber end cap can improve the damage threshold of the fiber end face. Then, the achromatic coupling optical system is designed in detail, and the best three piece achromatic glass combination ZNS/MGF2/IRG206 is selected for design. Finally, when the focal length of the coupling system is 35 mm, the coupling efficiency for 2.1-4.6 μm band reaches 92.74%. Further analysis shows that the optimal focal length range of the coupling optical system is 35-47 mm, and when the focal length is 40 mm, the maximum tolerance of optical fiber alignment is 60 μm.