Abstract: A long-span concrete filled steel tube (CFST) arch bridge, Longtan river bridge, is simplified into a space FEM model to calculate the dynamic responses to seismic an input. In the model, the boundaries between the foundations and the structure are considered to be connected with springs. The natural vibration frequencies are calculated with a space iteration program compiled by the authors, and three-dimensional geometric non-linear seismic responses are analyzed with Wilson-θmethod. Calculating results show that the interaction between founders and the structure influences frequencies of all orders considered, axial forces and moments, and mid-span displacements.