Though geophysical observation and numerical modeling suggest that the oceanic crust have been subducted along convergent continental margins into the deep Earth, the exhumed UHP metamorphic terranes of oceanic affinity are rarely exposed. The UHP metamorphic belt of China's southwestern Tianshan was inherited from the ophiolitic complex as a result of the northward subduction of the paleo-Tianshan ocean beneath the Yili-Central Tianshan plate. With the in-depth investigations of many aspects in recent years, the UHP metamorphism of China's southwestern Tianshan has been better understood. In this paper, the authors focused the discussion on the progress in petrological studies of UHP rocks from this orogenic belt. The ubiquitous discovery of coesite in meta-basaltic and meta-sedimentary rocks indicates that the meta-ophiolitic belt of China's southwestern Tianshan have reached the mantle depths with extremely low goetherms, which is consistent with thermodynamic forward modeling. The rarity of coesite and the development of various coesite pseudomorphs suggest the intensive retrograde overprint at low temperatures. Only in the unfractured domain of porphyroblastic garnet could coesite be preserved. On the basis of detailed petrographical observation and phase diagram model, the UHP metamorphism in the north of the metamorphic belt of China's southwestern Tianshan is confirmed and a coherent UHP unit (referred to as Kebuerte-Habutengsu UHP unit) is proposed. To the south of this unit, no UHP evidence has been identified in eclogite or pelitic-felsic schist. The imbricate sequence relation between HP and UHP units points to the northward subduction polarity of the paleo-Tianshan ocean. The progress in the study of petrology is fundamental for further unraveling the physiochemical processes of cold oceanic crust subducted to great depths and for constructing a more reasonable tectonic model of the South Tianshan orogenic belt in Central Asia.