Based on basic theory of the seismic response of a multi-degree-of-freedom spatial structure system, the spatial seismic responses of a long-span continuous steel truss flexible arch bridge under uniform and non-uniform excitations are analyzed using the dynamic time-history analysis method and spatial finite element model created by ANSYS software. Results of analysis show that the peak value of the arch rib's axial force and the main truss's bending moment appear around the arch foot and side of pier under non-uniform excitation. Further results are as follows:the maximum axial force at the arch foot and the in-plane bending moment under the multi-directional input of a seismic wave are 1.28 and 8.32 times greater than those under single-directional input, respectively; the axial force at the arch foot and the in-plane bending moment under non-uniform excitation are 2.5 and 8.4 times greater than those under uniform excitation, respectively; the peak value of lateral displacement at the arch rib under the a transverse direction input is 2.4 times greater than that under a longitudinal direction input; while the peak value of longitudinal displacement at the arch rib under the input of longitudinal direction is 2.6 times greater than that under the input of transverse direction. The bearing form also has some influence on the seismic response of structure. Results show the necessity of considering the space and time effect of seismic waves during the seismic design of long-span steel truss arch bridges.