A feature extraction algorithm for porphyrin sensor array (PSA) images is presented. At the first step, it achieves an automatic tilt correction for PSA images based on radon transform. Then automatic mesh generation is applied to PSA images based on the method of combining projection binarization and mathematical statistics. The areas of porphyrin spots are locating by local threshold segmentation for the images which have achieved mesh generation. Finally, feature values of porphyrin spots are extracted by radius ratio method from the porphyrin spot areas. The algorithm solves the problem that identification of weak signal spots is quite difficult, and the extracted feature values are the maximum point to reflect the information of porphyrin spots. By ammonia, isoprene, aldehydes and other gases tests, the results show that the algorithm can effectively extract feature values of porphyrin spots in PSA images.