Abstract: There is still a debate about whether the Jinshajiang Ailaoshan tectonic belt developed in the Late Paleozoic was a Paleo Tethyan Ocean or a backarc basin. This paper conducts petrographic and geochemical analyses of the Lower Devonian siliceous rocks in the southern section of the Ailaoshan tectonic belt in order to provide new evidence for the tectonic evolution of the area. The geochemical characteristics of siliceous rocks show that they are biogenic, with a significant negative correlation between SiO₂ and Al₂O₃, and SiO₂ and TiO₂, indicating that siliceous rocks do not come from terrestrial sources and have a negative Ce anomaly. The Ce/Ce * value is 0.73 to 0.76, with an average value of 0.75, which is close to the characteristics of siliceous rocks in ocean basins; The La_N/Ce_N value ranges from 1.31 to 1.49, with an average of 1.38, which is close to the characteristics of siliceous rocks in open ocean basins. Based on comprehensive analysis, it is believed that the siliceous rocks were formed in a transitional environment from marginal seas to open oceans, suggesting that the Jinshajiang Ailaoshan tectonic belt developed in the Late Paleozoic in the Paleo Tethyan Ocean rather than in a backarc basin.