Objectives: The tuff interlayer is widespread in the bottom of Middle Triassic, southwestern margin of the South China plate, and there exist different viewpoints about the provenance of the volcanic ash. The Youjiang basin is located in the southwest margin of the South China plate, deposited a suit of marine sedimentation during Early—Middle Triassic stage, which provides basis material for exploring the provenance of the volcanic ash. Methods: This article focused on the Early Triassic and Middle Triassic tuff in Gannan region, Zhebao, Youjiang basin, based on the section measurement, biostratigraphic analysis results divided the limestone into 4 conodont zones, the Neospathodus pakistanensis zone, the Neospathodus waageni—N. abruptus zone, the Neospathodus homeri—N. triangularis and the Chiosella timorensis zone, respectively, which determined the 17th bed tuff as the bottom of Middle Triassic. Results: The petrology analysis result shows that the tuff is subdivided into rhyolitic vitric tuff, belong to the alkali series, all samples are characterized by relative enrichment in light rare earth elements (LREE), obvious negative Eu anomaly, and displayed LILE (Rb, U, Th and Ba) enrichment and HFSE depletion (Nb, Ta, Ti and P), analogues to those of typical subduction- related geochemical. Based on the age of the tuff determined by the biostratigraphic and the isotope chronology, detailed comparative analysis through the regional distribution, the mineral composition and the geochemical characteristics of Middle Triassic boundary tuff in southwestern margin of the Yangtze plate, and the Lower—Middle Triassic volcanic rocks sequence features in Youjiang basin. Conclusions: Based on the age determine of the tuff by the biostratigraphic and the isotope chronology, combining with the regional distribution, the mineral composition and the geochemical characteristics of the Middle Triassic boundary tuff in southwestern margin of the South China plate, and the Early—Middle Triassic volcanic rock sequence features in Youjiang basin, detailed comparative analysis between the Middle Triassic tuff and the volcanic rocks in southwestern margin of the Yangtze plate and its adjacent region, and result show the Middle Triassic Pingxiang—Shiwandashan volcanic belt is the provenance of the lower Middle Triassic volcanic ash, Youjiang basin .