The strut-and-tie model (STM) approach for the reinforcement design of diaphragms in concrete box-girder bridges is studied in this paper. To begin with, an equivalent model for diaphragm is established by simplifying the boundary conditions. Based on the typical STM for a deep beam and on the load path method, the STM for a solid diaphragm and the diaphragm with opening are proposed. Further study about the STM for the diaphragms reveals that: as for the double-bearing diaphragm with depth-to-width ratios between 0.5―1.2, the lever arm between the horizontal tie and strut, the angle of inclined struts are dominated by the depth-to-width ratio and the spacing of bearings; as for the double-bearing diaphragm with the depth-to-width ratio between 1.2―2, the models can be assembled by a kind of prototype models for deep beams and diaphragm models with low depth-to-width ratios, which are no longer influenced by the spacing of bearings. Moreover, the variation of size and location of opening may disturb the load paths, and impact the inclined strut angles for the above models. Applying the proposed STM approach, the reinforcement design of diaphragms can be accomplished by virtue of the AASHTO LRFD specification. Finally, a worked example demonstrates that the proposed method can be a good practical tool.