South Tianshan orogeny is a typical orogenic belt between Tarim-Kalakumu plate and Yili-Kazakhstan plate. Kulehu-Keketiekedaban ophiolitic mélange is exposed in the south margin of the South Tianshan. Kaidu River section is located on the northern margin of Kulehu-Keketiekedaban ophiolite mélange in South Tianshan. In the section, Devonian volcanic rocks composed of ignimbrite, crystal pyroclastic-bearing felsic glassy lava and andesite were discovered. The Devonian volcanic rocks are of alkaline to calc-alkaline series, with A/CNK being 0.82～1.31. The ignimbrite is characterized by high content of REE and strongly enriched and highly-fractionated LREE, with low fractionated HREE and strong negative Eu anomalies. The LA-ICP-MS U-Pb age of zircons from ignimbrite is 393.1±3.4 Ma, suggesting that Saerming Formation belongs to Middle Devonian indeed. The age 393.1 Ma restricts the subduction time of the Kule Ocean basin. Kulehu Ocean basin started subduction in Middle Devonian (393.1 Ma), which released fluids and resulted in melting of mantle wedge to form basaltic magma. Thus, andesite was formed by fractionation and crystallization of the basaltic magma. When the crust was thickened, the basaltic magma continually ascended upward to cause the melting of the upper crust. Finally, the ignimbrite and crystal pyroclastic-bearing felsic glassy lava were erupted.