A lightweight optical system for helmet-mounted low-level-light night vision system was designed， including objective lens and eyepiece. In order to improve the imaging quality of the system while realizing the light weight of the optical system， the aspheric lens were employed in this design. Firstly， the lens parameters of the objective lens and eyepiece were determined according to the theoretical calculation， the proper initial structure was chosen， then， the aspheric surface was used for optimizing design， finally， a object lens with 7 lenses and an eyepiece with 2 lenses was presented， with the weight of the objective lens reducing form 35 g to 26 g， the weight of eyepiece reducing from 52 g to 20 g， and the weight of the total system reducing 41 g， the lens weight reducing by 47%， finally achieving the weight reduction of the system.