According to the basic principle of bridge indirect measurement, the finite element model of vehicle-bridge coupled vibration is established for a three-span continuous girder bridge. The time-history response of the vertical acceleration of the contact point is extracted when the vehicle passes the bridge at a constant speed. The first three frequencies of the bridge are identified by the peak picking method. The bandpass filtering technology is used to extract the component response related to the bridge frequency from the vertical acceleration response of the contact points, the first three modes of the bridge are obtained by Hilbert transform. The identified mode shapes are compared with the finite element theoretical mode shapes. It is show that MAC (modal assurance criterion) of the first three modes of mode recognition is greater than 0.99 when the vehicle speed is 4m/s and 8m/s. Based on the finite element model of the bridge, the mass of the identified modes is normalized, the test displacement flexibility matrix of the main girder is calculated, and the standard load test scheme of the bridge is designed. The flexibility matrix was used to predict the deflection of the main girder under the test load, and compared with the theoretical deflection. The results show that the errors of both the predicted deflection and the theoretical deflection are less than 5%, when the speed is 4m/s and 8m/s, which meets the requirements of engineering precision. This study provides an important theoretical reference for the further application of the indirect measurement method of vehicle-bridge coupling vibration in practical engineering