This paper carries out some geochemical analysis on a monzogranite in the Adenxile village, Sonid Zuoqi, Inner Mongolia. The age of this pluton is around 335.9±1.3 Ma according to the U-Pb dating by LA-ICP-MS. The analyzed samples are riched in SiO2, Na2O and K2O, but poored in MgO and TFeO. Their TFeO/MgO ratio ranges from 6.58~10.13. The Al saturation index (A/CNK) varies between 1.03 and 1.15. The LREE of these samples are highly differentiated, but the HREE present flat distributions. The element of Eu displays obvious negative anomaly with the δEu between 0.25 and 0.69. In the spider diagram, these rocks are highly enriched in Rb, Th and LILE and depleted in Sr, P, Ti, Nb and Ta. All the geochemical features above define this monzogranite as the highly differentiated peraluminum I-type granite, and they were formed in the settings related to subduction. This monzogranite, together with other Early Carboniferous arc-related felsic magmatic rocks in the Erenhot—East Ujimqin Banner region, constitute the magmatic arc caused by the northward subduction of Hegenshan Ocean.