It is chronically debated on the Permian tectonic evolution of the northern Qiangtang block, Qinghai—Xizang(Tibet) Plateau. Newly identification of the Late Permian quartz syenitic porphyries in the Tuotuo River area may provide some constraints on the PaleoTethyan tectonic evolution of the northern Qiangtang, even of the whole northern Qinghai—Xizang(Tibet) Plateau. The Later Permian porphyries, containing numerous basaltic and limestone xenoliths, intrude in the Early Permian andesites and limestones. They are calcalkaline, characterized by enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE) with depleted high field strength elements (HFSE) and significantly negative Eu anomalies. Sr—Nd isotopic data of the porphyries exhibit [n(87Sr)/n(86Sr)]ⅰ=0.70065~0.70566 and εNd(t)= 2.1~4.1 with TDM(Ga) = 0.679~0.828, suggesting the magma might have resulted from partial melting of the mantle wedge induced by dehydration of a subducted slab, e.g., the Late Permian quartz syenitic porphyries are arclike. The magma crystallized at shallow level under the surface with high crystallization temperature. During the ascending and intrusion, fractional crystallization and crustal contamination may have modified the rock's chemistry.Obviously, there must be a subduction of oceanic slab during the Late Permian along one margin of the northern Qiangtang block.