A　method　is　proposed　for　the　force　identification　of　bridges　under　operational　condition.　The　dynamic　stiffness　model　of　the　bridge　is　established　with　the　result　from　operational　modal　testing.　The　instantaneous　modal　amplitudes　are　obtained　by　processing　the　acceleration　time　histories　with　band-pass　filters.　Modal　forces　are　retried　both　in　time　and　frequency　domain.　Equivalent　static　modal　forces　are　computed　with　the　help　of　stochastic　vibration　theory.　Dynamic　amplification　factors　of　the　load　are　computed.　The　effectiveness　of　the　proposed　method　is　testified　by　an　experiment　carried　out　on　a　120　m　span　concrete　filled　steel　tubular　arch　bridge.