The high pressure(HP)-ultrahigh(UHP)metamorphic belt of southwestern Tianshan Mountains in Xinjiang mainly consists of pelitic-felsic schist that encloses lenses of eclogite,blueschist and ultramafic rock. The coesite-bearing pelitic schist shows porphyroblasitic/diablastic texture and schistose structure,and its mineral assemblage consists mainly of garnet,phengite,albite,paragonite,glaucophane,barroisite and quartz together with minor allanite,rutile and titanite. Coesite is included in garnet. The petrology and phase equilibrium modeling indicate that the coesite-bearing pelitic schist has experienced three metamorphic stages. Stage Ⅰ is the prograde to pressure peak stage characterized by increase in both pressure and temperature,which is constrained by garnet zoning from core to coesite-bearing domain. The p-T condition and mineral assemblage of the pressure peak are estimated to be 500℃, 2.9 GPa and garnet + glaucophane + jadeite + carpholite + lawsonite + rutile + phengite + coesite, which is comparable with the occurrence of coesite. Stage Ⅱ is the post pressure peak decompression and heating to the temperature peak,which is constrained by garnet zoning from coesite-bearing domain to rim. The p-T condition and mineral assemblage of the temperature peak are estimated to be 560℃,2.35 GPa and garnet + glaucophane + jadeite + lawsonite + rutile + phengite + quartz. The dehydration reaction glaucophane + carpholite + lawsonite=garnet + jadeite + H₂O releases ~45% of water bound in the rock and leads to coesite transforming into quartz and carpholite disappearance. Stage Ⅲ is the post temperature peak isothermal decompression stage, which is constrained by the p-T condition of equilibrium assemblage albite + paragonite + barroisite + titanite(495~550℃,1.0~1.15 GPa). During the decompression,the dehydration reaction jadeite + lawsonite=glaucophane + paragragonite + H₂O occurs at ~2.1 GPa,resulting in lawsonite decomposition and paragonite appearance. At~1.1~1.25 GPa,rutile is replaced by titanite, accompanied by omphacite disappearance as well as the formation of albite and barroisite. The strong continuous dehydration during stage Ⅱ is in great favor of re-equilibrium of mineral assemblage,which leads to most coesite at pressure peak transforming into quartz. Only minor coesite is survived in garnet. The coesite-bearing pelitic schist and its enclosing eclogite lens have experienced the same subduction and exhumation.