Abstract: The circular hole workpiece is widely used in aerospace and other precision manufacturing fields. Rapid and accurate detection of the inner wall image of the circular hole is an important means to ensure the quality of the workpiece. An image detection method of inner wall of circular hole workpiece based on 45° reflector was introduced. In this method, the sectional image of the inner wall was obtained by inserting the reflector into the workpiece, and the complete image of the inner wall was obtained by continuous rotation of the workpiece and image stitching ways. A 45° reflector was designed and manufactured for a workpiece with 4.5 mm inner diameter, and the effective imaging area as well as the influence of translation and tilt errors were analyzed theoretically. A measuring device for the inner wall of circular hole workpiece was built, and a special calibration workpiece was designed and manufactured to realize the calibration of pixel equivalent of the measuring device. The experimental results show that the system pixel equivalent is about 5 mm, the measurement relative error is less than 3.5%. The effective imaging area of a single image is obviously improved, and the complete image of the inner wall of the workpiece is obtained by the stitching method.