The effect of the width-to-span ratio on the mechanical characteristics of a large widened continuous box girder bridge is investigated. This study is based on the actual project of widening prestressed concrete continuous girder bridge for the expansion and interchange ramp of Changyi expressway in Hunan province. A grillage model and a single beam model for a continuous box-girder bridge with sharply variable width are established to analyze its bending moment, deflection and stress. The results show that under the combined effects of self-weight and secondary loads, as the width-to-span ratio increases, the moments borne by each longitudinal beam along the cross-section gradually become more uniform. However, the relative deviation in stress distribution among the longitudinal beams increases, indicating significant spatial effects. Compared to widening the main beam by increasing the width of the chambers, widening the main beam by increasing the number of chambers results in a more uniform distribution of deflection and bending moment.