In order to reflect the real status of an existing bridge, a new model updating method for multi-girder bridges was presented. In this method, the relative displacement in a hinge joint is supposed to be proportional to the shear force; the stiffness of hinge joint, flexural and torsional rigidity of girders are simultaneously taken as unknown parameters to be modified. The displacement equations are established with measured displacements on both sides of girders, and the optimal solution for the over-determined equations is obtained by the QR algorithm. The characters of multi-load cases, multi-loads and eccentric loads are considered in the proposed model, so that it can be used to bridge load tests. Numerical simulations for a multi-girder bridge with crash barriers and damages were carried out. The simulation results show that the proposed method can produce accurate corrected factors for girders with large damage or complex affiliated structures; load cases should be adopted as many as possible, and the loads should be arranged near the hinge joints or girders to be tested.