Abstract: The Mesozoic composite plutons widely distributed in South China are closely associated with rare metal mineralization. However, their genetic mechanisms remain highly debated. To constrain the petrogenetic model of such intrusions, this study focuses on the Jiuyishan composite pluton in southern Hunan, employing integrated whole-rock geochemistry, zircon U-Pb geochronology, and in situ Hf isotopic analysis. Zircon U-Pb dating yields weighted mean ages of 153.0 ±1.0 Ma, 153.1 ±0.9 Ma, and 153.8 ±1.5 Ma for the Shaziling, Jinjiling, and Pangxiemu plutons, respectively, indicating their emplacement during the early Yanshanian period. Whole-rock Sr-Nd and zircon Hf isotopic compositions suggest that these rocks were derived primarily from partial melting of ancient lower crust with minor mantle input, and are classified as A₂-type granites formed in an intraplate extensional setting. Based on Rayleigh fractionation modeling of the whole-rock Rb-Sr system, we propose a multi-stage crystallization differentiation model: initial crystal mush underwent melt extraction at 40%-50% crystallinity, with the residual cumulates forming the Shaziling pluton, while the extracted melt subsequently migrated and experienced further differentiation, eventually emplacing as the Jinjiling and Pangxiemu plutons. This model provides new constraints on the magmatic evolution of Mesozoic composite plutons in South China and their implications for rare metal enrichment.