With the rapid development of infrastructure in China, many small and medium-span bridges have been in service for over 20 years. Currently, the assessment of the health status of small and medium-span bridges, which relies on manual inspection, suffers from being time-consuming and labor-intensive. Using the response of a vehicle crossing a bridge as the information carrier of the bridge's dynamic characteristics for damage identification avoids the high cost, traffic interruption, and operational complexity associated with measuring the bridge's modal parameters via dynamic load testing. Based on the coupling between vehicles and bridges, the acceleration at the contact acceleration response is obtained using the centre-differential method. Variable modal decomposition is then employed to obtain the modal responses of the bridge at all orders, and driving modal components are used for damage identification. Taking a simply supported girder bridge as an example, the recognition ability of the damage index for single and multiple damages is investigated, and the damage degree and location are explored. The results indicate that that the proposed damage index can accurately identify each damage. However, when the damage degree is small, the presence of stray peaks can interfere with the accurate judgement of the damage location.