The Mahonondor granites, located in East Ujimqin Banner of Inner Mongolia, are predominated by monzogranite and syenogranite. SHRIMP zircon U-Pb dating shows that the crystallization age of the monzogranite is 307.0±1.9 Ma and that of the syenogranite is 299.7±5.3 Ma. The ages which are between the late Carboniferous and early Permian are consistent with the late Paleozoic magmatic activity age. Geochemical researches indicate that the granite from Mahonondor area in the East Ujimqin Banner of Inner Mongolia is rich in silica (SiO₂＝65.65%～71.86%), alkali (K₂O＋Na₂O＝9.59%～9.90%) and K (K₂O/Na₂O＝1.00%～1.73%), suggesting a typical high-K calc-alkaline to shoshonitic series with the metaluminous feature (A/CNK＝0.96～1.04). The rocks are poor in rare earth elements (∑REE＝82.60×10^-6～157.70×10^-6), enriched in LREE and LILEs (Rb, Th, K), and depleted in HFSE, especially Nb, Ta, Sr, P and Ti. Similar REE distribution patterns indicate that the Mahonondor granites have the same magma source. The δEu values vary from 0.44 to 0.87, with most rocks having moderate negative Eu anomalies. In addition, massive structure and miarolitic structure point to post-collisional granites. In a word, these characteristics suggest that these high-K calc-alkaline granites in the Mahonondor area were emplaced during post-collisional conversion from compression to extension, and were derived from fractional crystallization of medium-K calc-alkaline diorite-porphyrite, causing collision between the Sino-Korean and Siberian plates during late Devonian to early Carboniferous period. It is shown that the conversion time of the late Paleozoic orogenic process in East Ujimqin area was from 320.1±7.2 Ma to 307±1.9 Ma, the period 286～276 Ma was the developmental stage of post-collisional alkali granites, and Hegenshan oceanic basin had disappeared before 320.1±7.2 Ma. The above results provide new constraints on Mongolia Orogenic Belt tectonic-magmatic evolution in Late Paleozoic and closure of the Paleo-Asian Ocean.