The Yajiangqiao granitic pluton is important metallogenic granite in eastern Hunan. In order to accurately determine the emplacement age and petrogenesis of the Yajiangqiao pluton, LA- ICP- MS zircon U- Pb dating, geochemistry, and Lu- Hf isotopic analysis were implemented for the medium- coarse grained biotite monzonitic granite and medium- fine grained biotite monzonitic granite of the Yajiangqiao pluton. LA- ICP- MS U- Pb zircon dating yielded two ages of 213. 0±1. 2Ma（n=12，MSWD=0. 67）and 212. 1±1. 2Ma（n=14，MSWD=0. 09）for the Yajiangqiao biotite monzonitic granite. The two granites have SiO2 of 70. 39%~72. 88% and 73. 19%~74. 85%, and K2O+Na2O of 6. 94%~7. 95% and 7. 58%~7. 96%, with K2O/Na2O ratio of 1. 25~1. 70 and 1. 45~1. 58，and A/CNK ratio of 1. 05~1. 07 and 1. 07~1. 09, respectively. The chondrite- normalized REE patterns of the granites show are right oblique, distinct fractionation between LREE and HREE, with (La/Yb)N of 13. 76~24. 52 and 7. 12~11. 01 respectively, and moderate to obvious negative Eu anomaly, with the δEu value of 0. 34~0. 61 and 0. 26~0. 33. They are enriched in large ion lithophile elements (e. g. , Rb, K and Pb) and depleted in Ba, Sr, Eu and high field strength elements (e. g. , Nb, Ta and Ti). The εHf(t) values range from -7. 01~-0. 90 and -6. 38~+6. 85, and the two- stage Hf model ages (tDM2) are 13. 1~16. 9Ga and 12. 7~16. 6Ga, respectively. The results above demonstrate that the biotite monzonitic granite was the product of late Indo- China magmatic activity, suggesting that they may belong to weak peraluminous granite. The magma derived mainly from the remelting of Proterozoic crust, likely involved with a minor amount of mantle material. Combined with the previous results that the biotite monzonitic granite formed in a post- collisional extension setting, this study concludes that the granitic pluton formed from the emplacement and crystallization of granitic magma due to crust thinning and fast pressure reducing, along with uprising of mantle material.