Southeast Hunan is divided into two tectonic units by the Indosinian primary subduction fault (Chaling_Chinzhou fault), i.e., Yanling-Rucheng upwarping region to the east of this fault, and Hengyang-Guiyang downwarping region to the west. The Indosinian granitoids are high-K calc-alkaline peraluminous on the whole, and can be divided into two groups. The first group occurs in the NW-trending faults of Hengyang-Guiyang downwarping region, and is composed of hornblende biotite granodiorite, hornblende biotite monzogranite, biotite monzogranite and two-mica monzogranite; the rocks have relatively low contents of SiO2 with an average of 69.68%, and are mostly weak peraluminous with an average ASI value of 1.08 (0.96～1.29); they have comparatively low ISr values (0.708?0～0.716?8 ) and t2DM ages (1.48～1.72 Ga), high εNd(t) values of -8.90～-5.83. The second group lies in the Yanling-Rucheng upwarping region and Jiangjunmiao sub-upwarping region of Hengyang-Guiyang downwarping region, and is composed of fairly acidic biotite monzogranite and twomica monzogranite, with an average SiO2 content of 72.23%; the rocks are strong peraluminous with ASI valuesof 1.01～2.60 (averaging 1.30), and have rather high ISr values (0.715?9～0.734?5) and t2DMages (1.81～1.97 Ga) and low εNd(t) values (-12.0～-9.85). According to these characteristics as well as lithospheric structure and A/MF-C/MF diagram, it is thought that the second group granites were derived from schist and gneiss of the middle crust, whereas the first group granites came mainly from rocks of the middle crust, with the probable participation of minor basic granulite from the lower crust and mantle, suggesting the underplatingof mantle magma. Based on multiple oxide and trace element-diagrams for discrimination of structural environments, geological characteristics and tectonic evolutional settings, the authors consider that the Indosinian granites were formedin a post_collisional tectonic setting of the syn_orogenic stage, and the thickening of the crust and the subsequent weakening of the stress caused the melting of the middle crust, with magma formed sequentially. The underplating of mantle magma caused lower crust and mantle materials to enter the granitic plutons in the downwarping region and deep faults. Researches indicate that the Early Yanshanian granitoids were formed in a post-orogenic tectonic setting, and the Indosinian and Early Yanshanian tectonic settings restricted each other.