Abstract: For the deep holes, tiny holes or blind holes within deep cavity, due to the limitation of the size, shape and structure of cavity, it is very difficult to detect the inner surface and the aperture location information in the cavity.Aiming at this problem, an endoscope optical probe for detecting the small hole features and state in cavity was designed.This probe is a small diameter endoscope which can achieve 90 ° angle of view, 20 mm large depth of field with the large magnification of -0.71^×, the maximum distortion of 0.5%, and the full field-of-view modulation transfer function(MTF)near 0.1 at 166 lp/mm.The aperture detection accuracy is 0.01 mm, the resolution is 0.005 mm, and the optical system outer diameter is less than 5 mm.It can be used for detecting small aperture shape and location information in cavity parts with the cavity diameter range of 15 mm~45 mm and the aperture diameter range of 0.01 mm~4 mm.