In view of the problems of the short pupil distance and poor imaging quality of traditional eyepiece systems, an aspheric long interpupillary eyepiece with an interpupillary distance of 90 mm and a relative lens distance of 3.3 is designed to adapt military sighting devices for combat needs. First, based on a refractive eyepiece optical system composed of a spherical surface, an aspheric design is introduced to further improve the imaging quality of the eyepiece system and simplify the system structure, and a refractive long interpupillary eyepiece containing an aspheric surface is obtained, which consists of five lenses. The results show that at a spatial frequency of 50 lp/mm, the Modulation Transfer Function (MTF) of the entire field of view exceeds 0.4 for long interpupillary eyepieces with an aspheric design. The tolerances and imaging quality of the eyepiece were analyzed; the tolerances are reasonable, and the image quality is excellent. The total length of the optical system is 48.3 mm and the weight is 129.2 g, whereby the length was shortened by 24.2% and the weight reduced by 35.7% compared with a traditional refractive eyepiece. Therefore, being lightweight, it has the characteristics of a compact structure, and exhibits a high image quality.