In recent years，endoscopes have been widely used for the detection of internal defects in pipelines in various environments. This article focuses on the high-precision defect detection requirements of different calibers of gun barrels，a wide-angle endoscope lens with a large field of view and short focal length is designed. This lens achieves panoramic imaging of the internal walls of gun barrels within the caliber range of Φ76 mm to Φ155 mm，determines the position of internal defects，and allows the entire lens to rotate for side-view imaging of different areas of the inner walls，enabling high-precision measurement of defects. The system is designed using the Zemax design software based on the concept of a retro focal lens. Non-spherical surfaces are utilized to improve the imaging quality of the system. The final optical system achieves a field of view of 120°，a focal length of 2.5 mm，a total length of less than 30 mm，a maximum distortion value of less than 15%，and a full field of view greater than 0.3 at a detector cutoff frequency of 84lp/mm. The optical system has a simple structure，a wide applicable caliber range，and good imaging quality，meeting the requirements for gun barrel defect detection.