Based on a spatial grillage model, the moving load on a girder bridge is identified. According to the modal assurance criterion (MAC) matrix,the sensor placements were optimized using the successive accumulation method. The numeric examples were carried out for vehicle-bridge interaction identification on a tri-cell continuous concrete girder. The simulation results show that the optimal sensor placement can help to improve the identification accuracy and efficiency and reduce the susceptibility of identified loads to the eccentricity.