Shengsi Island, located at the northernmost end of the Zhejiang-Fujian magmatic belt, represents a transitional zone between land and sea, significantly influenced by the subduction of the ancient Pacific Plate. Petrological analysis, zircon U-Pb geochronology, zircon Lu-Hf isotopic microanalysis, and geochemical characterization were conducted on the Shengsi syenite granite pluton in this study.The Shengsi pluton primarily consists of potassium feldspar (50%-65%), quartz (20%-25%), plagioclase (20%-25%), and biotite (5%-10%), exhibiting a medium- to fine-grained granite texture. High-precision zircon U-Pb dating yielded a weighted average age of 95.4±1.2 Ma, indicating that the pluton formed during the Late Cretaceous. Zircon Lu-Hf isotopic analysis revealed the consistently negative εHf(t) values (-6.2 to -4.0) with a narrow range of variation, significantly higher than the average crustal value. The two-stage zircon model ages range from 1405 Ma to 1547 Ma (mean 1465 Ma), which are younger than the second-stage model ages of the Huaxia basement metamorphic rocks (Taoxi). Geochemical analysis indicates that the Shengsi syenite granite is characterized by high silicon, alkali, aluminum, and potassium contents, with extremely low levels of Mg, Ca, P, Ti, and Fe. It belongs to the metaluminous to weakly peraluminous, high-potassium calc-alkaline granite series. The rock is enriched in large-ion lithophile elements (e.g., Rb, K) and high-field-strength elements (e.g., Th, U, Zr, Hf), while elements such as Nb, Sr, and P are depleted. The total rare earth element (REE) content is relatively high, with stronger fractionation of light REEs compared to heavy REEs, pronounced negative europium anomalies, and high fractionation indices. The low Sr and high Yb content of the granite suggests relatively low pressure during its formation. The Shengsi syenite granite is classified as A-type granite based on the petrological and geochemical characteristics, as well as the tectonic setting. It formed during the Late Cretaceous in a tectonic environment associated with the retreat and extension of the ancient Pacific Plate. This process involved lithospheric thinning and crustal melting caused by the intrusion and upwelling of mantle material. The resulting magma, primarily crustal in origin, formed through shallow crustal intrusion after crust-mantle mixing.