The invention discloses a multi-beam image sonar-based real-time image processing method. According to the method, the multi-beam forward looking sonar image data of an underwater unmanned untethered submersible vehicle (AUV) are processed in real time, so that the real-time underwater obstacle information is provided for the AUV. In this way, the underwater obstacle-avoiding function of the AUV is realized. According to the technical scheme of the invention, based on the principle of the image sonar imaging technique, a large amount of noises and noise speckles in sonar images are filtered out through the dynamic distance attenuation threshold method filtering process and the enhancement algorithm. Meanwhile, the signal-to-noise ratio of images is improved. Based on the fuzzy clustering and morphology algorithm, the obstacle information in sonar images is filtered out from the sonar images. Therefore, the algorithm of the invention is stable, reliable and good in real-time performance. Based on the method, the obstacle information during the navigation process can be accurately and effectively provided. Furthermore, the underwater safety of the AUV is improved.