Detecting micrometer-sized holes in the skin is a major problem in the NDT of stratospheric airships, and to the best of our knowledge, there are still very few researches so far on NDT methods for microporous defects in envelope materials. In this paper, we propose to accomplish the automatic detection of microporous defects using an improved network based on U-net. While using a laser beam as a heat source, three different laser beam extension methods were compared in order to improve the problems of uniform laser beam intensity distribution and small heating area, and a fiber laser beam shaper with excellent heating effect was chosen as the experimental method. Micron-sized holes of different sizes were produced in the epidermal material. The detection of pore defects with a minimum diameter of 75 µm can be accomplished using the improved network with an accuracy of 95.3%. The experimental results show that the proposed method is able to significantly detect debonding defects at the adhesive bond as well as pore defects at millimeter and micrometer scale.