Abstract: The arch feet are the key sub-structure of tied arch bridges. In this paper, the multi-scale modeling method is introduced to the mechanical behavior analysis of the arch feet on tied arch bridges. To solve the problem existing in the conventional local-elaborate finite element (FE) model, this research introduces the flexible Boundary Conditions to substitute the hard Boundary Conditions used in the local-elaborate model. By employing the flexible Boundary Conditions, the global scale model of the arch bridge is added to the local-elaborate FE model by means of proper connection methods. Furthermore, the multi-scale modeling method is identified by the global-elaborate method. Based on the proposed multi-scale modeling method, a multi-scale FE model of a concrete-filled-steel-tube arch bridge with a steel truss and tie beams in Dalian city is established using ABAQUS. Simultaneously, a local-elaborate model and a global-elaborate model are established according to the local-elaborate and global-elaborate modeling methods for comparison and identification, respectively. Intensive comparison study demonstrates that the FE analysis result of the local-elaborate model is significantly influenced by the Fuzzy Region, and that the Fuzzy Region cannot be quantitatively determined, leading to unreliable analysis results or large amount of elaborate FE modeling, analyzing and data-processing work. Based on the idea of the flexible Boundary Conditions, the multi-scale modeling method can accurately simulate the complicate boundary conditions of the local sub-structure, and predict the mechanical behaviors of the arch feet without the influence of the Fuzzy Region. The results of the multi-scale model meet well with those of the global-elaborate model.