Abdominal fluorescence endoscopy significantly enhances the recognition effect of lesion tissue contours in minimally invasive surgery. However, most intra-abdominal endoscopic optical systems are designed with lens materials that consider only visible light transmission and imaging. Additionally, the existing laparoscopes are designed for adult surgery, typically exceeding 5 mm in diameter, with few options for small-diameter laparoscopes. To address this issue, this paper presents the design of an optical system for a small-diameter intraperitoneal fluorescence endoscope. The material selection considers the optical transmission characteristics of the visible and fluorescent bands. The designed system has an entrance pupil diameter of 0.1 mm, a diameter of 3 mm, a distortion of less than 22.7%, and a full field of view of 90°. At an object distance of 10 mm, the proposed system achieves center resolutions of 14 and 9.9 lp/mm for visible light and near-infrared fluorescence, respectively. The tolerance analysis results indicate that the designed system meets the actual usage requirements.