Abstract: In the fluorescent penetrant testing (FPT) of surface defects on aerospace parts, the fluorescent indications are normally observed and evaluated by visual inspection or magnifying glass, then measured and graded with fluorescent comparison scale. However, observation with magnifying glass is limited by insufficient resolution, and it could not realize the recording and preservation of fluorescent images; the fluorescent comparison scale could only provide the size range of fluorescent indications, with obvious limitations in measurement accuracy. Based on the imaging principle of biological fluorescence microscopes, this paper optimizes and reforms the optical path system of a metallographic stereomicroscope. By integrating high-brightness light-emitting diode (LED) ultraviolet light sources and installing additional filtering devices at the light source, objective lens and camera interface, an ultraviolet/white light dual-source microscope is successfully developed. This microscope has enabled the simultaneous observation and collection of fluorescence images and white light images of in weld seam crater and cavitation defects on the inner bore of hydraulic housing, as well as the regular testing of standard crack specimens. It could precisely measure the multi-dimensional dimensions of the defects with measurement accuracy of ±0.01 mm, which effectively solves the technical problems in qualitative evaluation and quantitative measurement of fluorescent penetrant testing.