Abstract: Widening existing bridges with steel-concrete composite structure can effectively shorten the construction period, reduce the beam height and the number of piers. The comprehensive benefit is obvious in busy traffic junctions of the city. In this paper, steel-concrete composite structure is used for widening design of existing concrete elevated bridges based on practical engineering, beam-shell finite element model is established to analyze mechanical properties of widened bridges, and overall mechanical performance and working state of bridges are studied by load tests. The results show that the overall working state of bridges widened by steel-concrete composite structure is good, and the measured results are close to the theoretical analysis model. The structure system is safe and reliable.