Abstract: In the survey of oceanic polymetallic sulphides, detailed geological mapping is needed for a known hydrothermal area, which often covers a small area, contains less geological elements, and is not connected with regional tectonism and magmatism. Based on the data of deep-towed optical system obtained from ocean voyages over the years in the Segment 26 (51°E) of the Southwest Indian Ridge (SWIR), and combined with high-precision multi-beam bathymetric data, we proposed a systematic division principle of substrate hydrothermal anomaly, and identified four types of substrate hydrothermal anomaly: hydrothermal altered rocks or breccia, suspected hydrothermal sediments, hydrothermal organisms and their remains enrichment, and cemented carbonate rocks. The results of high-resolution submarine geological mapping show that there are 4 abnormal areas of hydrothermal activity in Segment 26. The hydrothermal activity occurrence rate of Segment 26 ranged 2～10, which is at least 1.8 times higher of the global one using empirical formula. Therefore, there may be more submarine hydrothermal activities in Segment 26 with melting anomalies or non-transform discontinuities of ultra-slow spreading ridge, and it also has the potential to form large polymetallic sulfide deposits.