Abstract: In recent years, the endoscope is widely used in defect detection of small-size parts in complex environment. An industrial endoscope optical system was designed for aero-engine blade detection. The basic structure of the lens adopted secondary imaging, and the objective lens adopted asymmetrical inverse distance structure, which beamed the large field of view light into the small caliber cavity. The adaptor lens was adapted to the high speed camera in which the diagonal length was 42 mm, and it could magnify the intermediate image 21 times. The system was based on Zemax design software for system optimization, tolerance analysis and image quality evaluation, and finally the system realized the optical properties of large field of view (120°), small caliber (3 mm) and high temperature resistance (25 ℃～180 ℃). Due to the higher requirements of the field of view, caliber and adaptor magnification, the aspherical surface was reasonably introduced to improve the imaging quality of the system. The entrance pupil diameter was increased to 0.5 mm, the system spatial cut-off frequency was at 17 lp·mm⁻¹, the full-field modulation transfer function value was greater than 0.28, and the maximum distortion value is less than 21.2%.