Objective To investigate the detection system and the precision measurement technology applicable for the small size fire hole of cartridge case. Methods A typical machine vision hardware and a dedicated bench were built. After pixel-level edge detection using Canny operator and Otsu algorithm, polynomial fitting along the gradient direction was used to precise positioning edge profile, and the corresponding program was compiled. Multiple measurements were performed with the same and a plurality of holes for comparison tests. Results The detection accuracy of the system could reach 5 μ m. The measurement process, with a high reproducibility, was not affected by subjective factors. Conclusion The system could meet the inspection accuracy specifications of ± 0.01mm, and the technique could be used for precise measurement of small pore size.