Top 10 natural frequencies and mode shape characteristics of four types of light-rail box girder bridges were summarized by FE modeling. And the mode analysis results were horizontal, vertical contrasted. Furthermore, the relationships between the natural frequency and structural stiffness were studied. Finally, the bridge-types of four types of light-rail box girder bridges were optimized. The results show that the torsional stiffness of the box girder with side walls and diaphragms is much greater than that of the box girder without side walls and diaphragms, the torsional stiffness of the single-box-double-cell box girder with eccentric-hole diaphragms is greater than that of the single-box-double-cell box girder with center-hole diaphragms, and the single-box –double-cell box girder with side walls and eccentric-hole diaphragms is the optimal bridge type of all light-rail box girders.