Tianchi volcano located on the China/North Korea border is a composite volcano with typical bimodal suites (trachybasalt shield, a trachyte stratocone and a comendite ignimbrite sheet). The petrogenesis of trachyte and the relationship between the mafic and the felsic suite are ambiguous. In this paper, we combined both new and literature data examined the petrogenesis of Tianchi felsic rocks and developed a possible petrological and geochemical model. We found that Tianchi trachytes can neither be formed by partial melting of the old lower crust of the North China craton nor by partial melting of the newly intruded basalts. The results of qualitative and quantitative geochemical modeling suggest that the felsic rocks are formed by fractional crystallization from the shield basalts with assimilation the crust. In addition, at least two different evolve paths exist in Tianchi felsic rocks: the high and low 87Sr/86Sr path. We believe that the two differentiation paths are related to the development of crustal plumbing systems. In the early stage, the evolved magma directly encountered with the upper crust, therefore, had high 87Sr/86Sr. In contrast, at the late stage, the differentiation magma evolved along the low 87Sr/86Sr trend since the fresh magma was isolated from the upper crust by the previous cooled magma body.