An optical system of handheld low-light level night vision sight based on customer specifications is designed, which can meet the needs of day and night. At first, the optical parameters are calculated according to the customer's specifications, the complementary metal oxide semiconductor (CMOS) detector and the display screen are selected. And then, the low-light objective lens and eyepiece are designed. At last, the final image quality evaluation results show that the optical system has good imaging quality and can meet the actual engineering needs.