The Dongqiyishan alkali feldspar granite is the largest rare metal mineralized granite in Beishan area, Iinner Mongolia, and is of remarkable representative significance in similar rock masses. Based on previous research result of petrologic and metallogenetic characteristics, the authors performed a lot of detailed work on geochemistry, isotope geochemistry and geochronology. Petrochemical research shows that the granite is rich in silicon and alkali, and poor in Fe, Ti, Mg and Mn, thus belonging to high-k calc-alkaline series. Petrochemical research shows that the granite is strongly depleted in Eu(δEu＜0.1), Ba, Sr, P, Ti and obviously enriched in Rb, Nb+Ta, W, Mo, Bi, Li, etc. The Rb-Sr isochron age is 128.2±1.1 Ma, with I_Sr being 0.709 79, suggesting that the original rock had obvious crustal characteristics. Relative to values of the depleted mantle, the two types of rocks have somewhat higher ¹⁴⁷Sm/¹⁴⁴Nd values and lower ¹⁴³Nd/ ¹⁴⁴Nd values, and the Nd isotopes of granites are similar to things of Beishan area granites. According to lithological characteristics and formation environment, the authors hold that the Dongqiyishan alkali feldspar granite was formed by the magma derived from partial melting of the upper crust through violent differentiation evolution in the late Yanshanian extrusion-tension environment. First, the alkali-feldspar granite porphyry was formed and constituted the exterior facies of the porphyraceous alkali feldspar granite. The formation environment of the alkali-feldspar porphyraceous granite was relatively closed; it was formed through the further enrichment of the volatile components and rare elements and a slow process of crystallization differentiation. Therefore, the self-metasomatism produced by the residual fluid became more developed, resulted in more close connection of lepidolite mineralization and secondary albitization and silicification to niobium-tantalum, tungsten, tin, rubidium and other rare metals mineralization The results obtained by the authors have some guiding significance in the study of Mesozoic granite-related metal ores of Yanshanian period, especially in the prospecting for niobium-tantalum, tungsten, tin, rubidium and other rare metal deposits.