free vibration analysis of long span bridge is the base of the earthquake and wind resistance design of bridge structures. It's also the base of the analysis of dynamic interaction response between bridges and trains. The frequencies of bridge structures arc the key parameters reflecting the free vibration features. In this paper，a long-span suspension bridge scheme is selected as calculating example，and the modal analysis is then performed to determine natural frequencies and mode shapes of lateral，vertical，torsional，longitudinal， and coupled vibrations of the bridge.