This paper aims to put forward a new method of strengthening the existing bridges with narrow decks and low bearing capacity to overcome the high cost and pollution problems in dismantling them. The method, strengthening bridges by built stringers and cross beams, is by widening some parts of a bridge. Based on the basic theory of finite element, we have set up an analysis model of strengthening the bridges by widening, which helps to demonstrate the rationality of this method. Meaningful conclusions have been drawn from the parameter analysis of the rigidity of the new widening stringers, the rigidity of the new cross beams and the relative settlement of the new and the old foundations. This method can be referenced by the reinforcement and maintenance of medium and small bridges.