The　effect　of　geometric　nonlinearity　on　lateral　seismic　response　of　long　span　steel　arch　bridges　is　analyzed,　and　the　seismic　response　of　lateral　bracings　and　its　influence　on　arch　ribs　are　paid　special　attention　to.　The　result　shows　that　lateral　bracings　are　the　weakness　of　structure　under　the　lateral　excitation,　and　are　most　likely　to　be　damaged　before　the　ingoing　of　nonlinearity　of　arch　ribs,　which　may　decreases　the　lateral　stiffness　and　therefore　reduces　the　lateral　stability　of　the　steel　arch　bridge.