Abstract: Through the comprehensive research on major elements, trace elements and rare earth elements of the clastic rocks in Upper Triassic Baiguowan Formation of Xichang Basin Sichuan Province, this research found that the major elements are rich in SiO₂, CaO and P₂O₅ , and lack of Al₂O₃, TFe₂O₃, MgO, Na₂O and K₂O. The trace elements are rich in Zr, Cs, Th and U. The capacity of rare earth elements are relatively higher. The fractionation of light and heavy rare earth elements is obvious. The light rare earth elements is richer than the heavy ones, and have medium negative europium anomaly. The provenance of Baiguowan Formation in Xichang Basin is felsic rocks and passive continental margin area. The climate period experienced the process from humid to arid and to humid and arid again. Compositional maturity of Baiguowan Formation rocks is generally low, and shows the variation from high to low, and to high and low again. The variation is negatively correlated with chemical alteration index(CIA). This paper provides basic data for the study of environmental evolution of Late Triassic.