The large Penglai 9 1 oilfield in the Bohai Bay Basin is the first discovered buried hydrocarbon reservoir of the Mesozoic granite. The geochemical characteristics and origin of the rocks provide important clues for the mechanism of Mesozoic tectonic environment of and continental crust growth of Bohai Bay Basin. A new age of 164±2 Ma was obtained for the Penglai 9 1 granitic pluton by LA ICP MS, and represents the formation age of the pluton consisting mainly of quartz monzodiorite，granodiorite and monzonitic granite．The study of major elements shows that the ω（SiO2）=6222%~7086%,ω（K2O）=329%~436%,ω（CaO）=171%~410%, AR=213~306，A/CNK=089~107, indicating that the pluton belongs to I type granite of high k calc alkaline series. The chondrite normalized REE patterns show a right dipping mode, enrichment in LREE and LILE, depletion in HREE and HFSE. The geochemical characteristics of the subduction zone components (SZC) display its close relation to the source region of magma. The granite is characterized by high contents of Sr, Sr/Y and La/Yb, and low contents of Y, Yb and Mg#, showing characteristics of adakite rock resulting from partial melting of thickening lower crust．The R1 R2 discriminant diagram and (Y+Nb) vs Rb tectonic diagram reveal that the granite was formed in the post collisional expansion stage. In conclusion, the Penglai 9 1 granite pluton were formed by partial melting of lower crustal materials including subduction materials in the crust thickening background due to the postcollisional extension. Its formation marks that the main collision of the Mesozoic continental convergent collision in the Bohai Bay basin had ended and entered post collisional extension stage from the compression of the main collisional stage during the period of 164 Ma, providing new evidence to further determine the Mesozoic continental collision process in the Bohai Bay basin in detail.