Abstract: The light aperture of the ultra-fine endoscope is fine, at presents most of the ultra-fine endoscopes in the image transmission system use image transmission fiber, which is expensive, and the image transmission has defects such as moire. An ultra-fine endoscope was designed in this paper. The image transmission system consists of a number of different short rod lens with a diameter of 0.8 mm and a length of less than 6 mm. The working length of the endoscope is 121 mm, the full field of view is 80°, the radial angle is 0°, and it works in the visible light band. The lens used in the entire endoscopic optical system is a spherical mirror, the modulation transfer function (MTF) of image resolution is greater than 0.1 at 113.5 lp·mm⁻¹, which has a higher resolution than the endoscope with the same diameter image transmission fiber (082F1000-7.0), the diffraction limit could be reached, the depth of field covers the full working range, and all kinds of aberrations are optimized. After the prototype assembly test, the image is clear and the resolution could reach the design index. By using rod lens instead of image transmission fiber, this design provides a feasible solution to solve the problem of moire pattern and high cost when using image transmission fiber for ultra-fine endoscope.