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.