Abstract: This paper presents a comprehensive theoretical analysis of an actual concrete-filled steel tubular (CFST) arch bridge under tridirectional spatial motions using the pseudo-excitation method (PEM). Formulations for modelling tridirectional spatial motions at varying sites are derived. Then, stochastic seismic analysis of the CFST arch bridge is conducted, by considering the dimensionality, incoherence effect, wave-path effect of ground motions, and local site effects with different and irregular site conditions. It is concluded that 1) the site effect can significantly affect structural responses; 2) the softer site conditions can amplify the larger structural responses; 3) the varying and irregular site conditions can induce significant variations of structural responses; 4) effect of dimensionality of earthquake motions and inclusion of the vertical ground motions should be considered in the seismic analysis to avoid erroneous response predictions.