Upper Permian-Middle Triassic sections in Youjiang basin contain many volcanic tuff horizons. Zircon LA-ICP-MS U-Pb dating of 4 volcanic tuff samples from the bottom of the Luolou and Baifeng formations in the Jiuxu and Lizhou areas, Guangxi indicate that episodical volcanism occurred during Middle Permian Gudalupian and Middle Triassic Bithynian in southwestern China. The petrographical study of the volcanic tuffs indicates the Youjiang basin experienced the tectonic transition from extension to compression after Middle Triassic, and especially after Bithynian, which resulted in the great tectonic subsidence of the basin.