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 120m span concrete filled steel tubular arch bridge.