The Miao'ershan pluton in the western segment of the Nanling Mountains is mainly composed of Caledonian granites with minor Indosinian and Yanshanian granites. The Caledonian plutons are composed of early stage biotite granodiorite, middle stage porphyritic biotite monzogranite (principal part) and late stage fine-grained biotite/two-mica monzogranite, with all the rocks having massive structure. The authors carried out U-Pb zircon dating of a porphyritic biotite monzogranite sample and a fine-grained two-mica monzogranite sample and obtained weighted average ages of 428.5±3.8 Ma and 409±4 Ma respectively, suggesting two magmatic events that occurred in the late episode of Early Silurian and in the late episode of Silurian to the early stage of Devonian respectively. Geochemically, the middle stage rock body is a high-K calc-alkaline peraluminous granitic pluton and is characterized by higher SiO₂ (70.09%～76.59%), moderate Al₂O₃ (12.71%～14.72%), higher K₂O (4.48%～5.73%) and moderate alkali (Na₂O+K₂O=7.24%～7.91%) content as well as higher ASI (1.15 on average). The granite is depleted in Ba, Nb, Sr, P and Ti but enriched in Rb, (Th+U+K), (La+Ce), Nd, (Zr+Hf+Sm)and (Y+Yb+Lu). It displays moderate total REE content (ΣREE=179.8×10^-6～270.6×10^-6), enrichment of LREE [(Ka/Yb)_N=5.07～14.33] and negative Eu anomalies (δEu=0.15～0.46). It also has slightly higher I_Sr (0.706 60～0.720 82) but lower εNd(t) (-8.29～-7.94) with Nd model ages (t_2DM) being 1.81～1.84 Ga. The C/MF-A/MF diagram indicates that the Caledonian Miao'ershan granites came from mudstones and clastic rocks. All these geochemical characteristics point to S-type granitiod and suggest that the granites might have come from rocks of middle-upper crust. Multiple oxide and trace element diagrams for discrimination of structural environment show that the granites were formed in a post-collisional tectonic setting. Based on petrogenesis and discrimination of structural environment in combination with regional tectonic evolutional setting, the authors infer that the formation mechanism of Caledonian Miao'ershan granites should be as follows: the thickening of the crust in Caledonian movement caused the rise of the crust temperature, and later weakening of stress in a post-collisional tectonic setting resulted in the melting of middle-upprer crust, with the magma emplaced in a relative open environment. These results and regional geological data indicate that there occurred strong intracontinental compression and crust thickening in Miao'ershan area in Beiliu movement as well as in Guangxi movement.