The Middle-Upper Triassic marine strata in the boundary area of Guizhou and Yunnan Provinces are famous for the well-developed marine reptile fossils and other palaeobiocenoses. In the Jiangdi section found recently by the authors,both Zhuganpo Formation and Xiaowa Formation are successive and well exposed. The representative Xingyi Fauna and Guanling Biota were found at the top of laminated limestone Member in Zhuganpo Formation and in the upper part of Xiaowa Formation,respectively. In order to certify the environment in which the biotic communities were living,97 samples from Jiangdi section were collected for both major and trace elemental analysis. According to the results,the Xiaowa Formation and the laminated limestone of Zhuganpo Formation were deposited in the oxgen-deficient environment. The oxgen content increased in the strata of Xingyi Fauna and Guanling Biota. However,in the interval nodular limestone between the two biotas,the environment might be anoxic. The coincidence of oxygen content change and the biotic communities evolution may indicate that the oxygen content played a certain control role in the biotic succession.