Waibeishan alkalifeldspar granite is mainly composed of perthite, albite, quartz, with minor amounts of iron biotite. There developed miarolitic structure, filled by quartz druse and potassium feldspar. In petrochemistry, the rock mass is rich in SiO2, K2O, FeO, F, while poor in Al2O3, MgO, CaO, P2O5. Riching in REE \[the variation range of REE is (107.4~330.71)×10-6\], with obviously stronger fractionation of light REE than that of heavy REE and conspicuous negative Eu anomalies. Besides, the “W”type tetrad effect is present in the granites’ REE distribution diagrams. In the aspect of trace element, the rock is rich in Rb, Th, Cs, U, Nb and poor in Sr, Ba, P, Eu and Ti, which displays the typical aluminous Atype granite characteristics. Zircon U-Pb LA-ICP-MS isotopic dating shows that the age of the rock mass is 96.2±0.8Ma. Compared with other Southeast Coastal Late Mesozoic Atype granite, Waibeishan alkalifeldspar granite bears significantly higher Nb/Ta and lower Y/Nb, which categorizes it as A1type granite. Comprehensively, the analysis indicats that the strong extension and fracture activation of early Late Cretaceous lithosphere in the southeast coastal area of China caused partial melting of mafic lower crust in poor water under high temperature and low pressure, which generated granitic magma. The Waibeishan Nbrich aluminous Atype granite was formed by the metasomatism of Frich fluid during the fractional crystallization process of the granitic magma.