The alternative selection of a bridge decides its performance in whole. The misplay in alternative selection of a great bridge project would result tremendous losing, which could not be covered during the subsequent detail design, construction, and operation stages. Therefore, it is necessary to discuss the basic principles of alternative selection for bridges. The concept of economical span length was firstly proposed, which reveal the economical principle for the layout of span number and length to the multi-span bridges, and also can explain the relationship between the technique enlarging the span lengths and that building the deepwater foundations. Some basic principles which should be followed in the alternative selection for girder bridges were presented through the comparisons of simple-supported girder and constant depth continuous girder bridges, and continuous girder and continuous rigid-frame bridges. Moreover, the reasons why cantilever and T-shaped rigid frame bridges had been replaced were discussed.