The Erchangfang plutons at the Keketuohai area in southern Altay orogenic belt are mainly composed of biotite granodiorite, and the authors studied their LA-ICP-MS zircon U-Pb ages and major and trace elements. The results shows that the zircon ²⁰⁶Pb/²³⁸U weighted average age is 398.0±3.5 Ma(MSWD=1.3), indicating that the plutons were formed in the early-middle Devonian period. The SiO₂ values range from 65.40% to 69.31%, σ from 1.27 to 1.65, and A/CNK from 0.92 to 1.02, suggesting intermidiate-K, calc-alkaline, metaluminous rocks. In addition, they are enriched in LILE (Cs, Rb, Th, U) and LREE, with obvious negative anomalies of HREE and HFSE (Nb, Ta, Zr, Hf) and Eu (δEu=0.48～0.65), thus resembling granitoid rocks formed in island-arc. Combined with the data obtained from the southern margin of Altay, the authors hold that the Erchangfang plutons were generated by partial melting of the lower crust underplated by mafic magma of the mantle in an island-arc setting of active continental margin, accompanied by magma mixing.