The Middle and Lower Triassic marine facies (including basin facies, slope facies and platform facies) sedimentary strata and multilayer tuff are widely distributed in Southwest China. Zircon LA- ICP- MS U- Pb dating was firstly carried out for the tuff from the Middle/Lower Triassic boundary of the Shawo section in the Qianxi area and Liuxi section in the Yiliang areas, yielding the boundary ages of 247. 2±2. 4Ma (2σ, MSWD=0. 85) and 247. 4±1. 4Ma (2σ,MSWD=0. 51) , respectively. The time limit for the Middle/Lower Triassic boundary in the Center Guizhou Uplift and the northern depression area was determined, and coincides with the age of 247. 2Ma for the Olenekian- Anisian boundary suggested by the International Chronostratigraphic Chart (2018 edition) and the TIMS age of 247. 2±0. 1Ma of the tuff near the Olenekian- Anisian boundary in Guandao section, Guizhou Province. The geochemical characteristics of tuff rocks show K2O and MgO enrichment and obvious Eu negative anomaly, indicating that the tuff may be derived from the continental magma arc formed by the ancient Tethys subduction. The episodic volcanism may be an important cause responsible for the intermittent deterioration of the ecological environment and the delayed biotic recovery in the Early- Middle Triassic. The result of this study will deepen the understanding of the Triassic episodic magma activity, and provides a basis for the Triassic stratigraphic division, tectonic evolution and exploration of the Triassic biorecovery- radiation and ecosystem co- evolution in the southwestern China.