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 is extracted when the vehicle passes the bridge at a constant speed. The acceleration spectrum is obtained by using fast Fourier transform. The acceleration spectrum is obtained by using fast Fourier transform. The central difference method is used to calculate the time-history response of the contact point acceleration. 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. The results show that the change of vehicle mass has no obvious effect on modal identification. Although the low speed is unfavorable to the mode identification, the accuracy of mode identification can be ensured by selecting the appropriate speed. 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 predicted deflection and theoretical deflection meet the requirements of engineering accuracy.