We present detailed petrography, geochemistry and zircon U- Pb isotopes of the Hailesitai granite porphyry in East Ujimqin Banner, Inner Mongolia, with the aim of determining its age and petrogenesis, important for understanding the early Paleozoic tectonic evolution of the Xing’an—Mongolian Orogenic Belt. Resluts: The Petrography feature suggest that Hailesitai pluton mainly consist of granite porphyry. Zircon U- Pb analyses yield ages of 463. 5±2. 0 Ma for the granite porphyry, indicating that it formed during Middle Ordovician time. The granite porphyry is characterized by high silica, alkali, low titanium and aluminum, lean calcium and magnesium. They are also enriched in large ion lithophile elements (e.g. Rb, Th, K) and high strength field elements (e.g. Zr, Hf) and strongly depleted in Sr, Eu, P, Ti and mediate depleted in Ba, Nb, Ta, with high 10000Ga/Al ratios and Zr+Nb+Ce+Y contents. These features indicate that Hailesitai granite porphyry is A2- type granite.Conclusion: The Saturation temperature of zircon ranges is between 878~899 ℃, and has a very low Sr and high Yb, indicating that the granite porphyry was formed in the extensional tectonic background of low pressure and high temperature and generated by partial melting of the thinning crust. Combined with regional geological background, we infer that the Hailesitai granite porphyry were formed in a back- arc extensional environment of an active continental margin which was resulted from northward subduction of the Paleo- Asian plate beneath the Siberia block along Sonid Zuoqi to Xilinhot.