Abstract: The Lower Devonian Xiangyangsi Formation is located in the west of Changning-Menglian suture belt at the junction of the Gondwanaland and the Pan-Cathaysian continent in Gengga, Changning, western Yunnan. The chert was discovered in the Xiangyangsi Formation for the first time, and the geochemistry of the chert was studied. Crinoidal stems with radiolarians and diatoms are distributed in chert. An analysis of the data of major elements shows that the values of SiO₂ are relatively high, being in the range of 85.43%-93.50%, the average ratio of Al/(Al + Fe + Mn) obtained in chert is 0.77, suggesting that the chert of the Xiangyangsi Formation is characteristic of biogenesis activity, along with some chemical activity. An analysis of the data of REE shows that the values of REE are 53.208×10^-6, δ Ce has a value of 1.02, and δEu has a value of 1.01. The average ratio of La_N/Yb_N and La_N/Yb_N is 0.99 and 0.84, suggesting the continental margin and deep-sea environment. With the rapid expansion of Changning- Menglian Ocean, a wide chert deposition took place due to the regional synsedimentary fault on the continental margin of Baoshan massif from Early Devonian. In combination with the characteristics of chert geochemical and the paleontological research, the authors hold that the Xiangyangsi Formation in Gengga is characterized by deep-sea deposition on the continental margin of Baoshan massif of Gondwanaland close to Changning-Menglian Paleo-Tethys Ocean.